[Ethereum] Transactions not getting mined if I send too many

I'm running a private network with one miner and I have the problem that if I send too many transactions from a node, those transactions are not getting mined.

For example: If I send one transaction every 6 seconds, then not a single transaction gets mined. If I send one transaction every 30 seconds then everything works fine.

I send a transaction with nodejs like this:

instance.myFunction(data, {from: account.address, gas: 4000000}, function(error, result) {
// do something
});

The transactions just pile up and not getting mined by the miner.

Here are for example two transactions:

> eth.getTransaction( '0x1e06fec8994ee38a32b2438d51b57c9d4bc95239c45322d58a4a9f0c8a03be4a')
{
  blockHash: "0x0000000000000000000000000000000000000000000000000000000000000000",
  blockNumber: null,
  from: "0x7ee9e416fdb371a9190dfa8fdf7361e66ada7e12",
  gas: 4000000,
  gasPrice: 18000000000,
  hash: "0x1e06fec8994ee38a32b2438d51b57c9d4bc95239c45322d58a4a9f0c8a03be4a",
  input: "0x4ee27fa900000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000006313530313331393039383234343b343000000000000000000000000000000000313530313331393039393234383b323400000000000000000000000000000000313530313331393130303235323b353500000000000000000000000000000000313530313331393130313235363b373700000000000000000000000000000000313530313331393130323236313b393100000000000000000000000000000000313530313331393130333236353b313800000000000000000000000000000000",
  nonce: 107,
  r: "0xe761ccc9db6d387eadba7bcb93dac6c582f8baf052da76eaff828e5b91c1722d",
  s: "0x571f99554f299d551bb05bb6145210a6480c015b42604f9bd885de6eea94dbcb",
  to: "0x8e195493424d416300f8b61b01145956345cd914",
  transactionIndex: 0,
  v: "0x42",
  value: 0
}
> eth.getTransaction('0x02e09d88360c27f6da92075a8ab1f8e2bb23bca149004a3d134048492348677d')
{
  blockHash: "0x0000000000000000000000000000000000000000000000000000000000000000",
  blockNumber: null,
  from: "0x7ee9e416fdb371a9190dfa8fdf7361e66ada7e12",
  gas: 4000000,
  gasPrice: 18000000000,
  hash: "0x02e09d88360c27f6da92075a8ab1f8e2bb23bca149004a3d134048492348677d",
  input: "0x4ee27fa900000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000006313530313331393130343331353b343000000000000000000000000000000000313530313331393130353331373b353700000000000000000000000000000000313530313331393130363332313b383500000000000000000000000000000000313530313331393130373332363b313500000000000000000000000000000000313530313331393130383332393b363000000000000000000000000000000000313530313331393130393333323b323000000000000000000000000000000000",
  nonce: 108,
  r: "0x183a1fc9c149ece66ed8e782758f7980d3b01c88595662e769171e155749dc4a",
  s: "0xfd9abed2168cae1e33c2bd4441015178e54aa3fb1c742f4f613d9d3c140ef4",
  to: "0x8e195493424d416300f8b61b01145956345cd914",
  transactionIndex: 0,
  v: "0x41",
  value: 0
}

I don't understand, why it's not working. I don't think one transaction every 5 second is a lot, they should get included into one block.

I'm using web3 version 0.20.0.

update:
to make things even more weird, it is sometimes actually working but sometimes it's not – seems like completely random to me. I can see in the console that the transactions are sent, I can also see, that new blocks are generated and synced with my node – once it stopped working, I have to restart the node.

update:
I tried to override the nonce manually but that didn't helped. All transactions are just queued in the transaction pool:

> txpool
{
  content: {
    pending: {},
    queued: {
      0x7ee9e416fdb371a9190dfa8fdf7361e66ada7e12: {
        110: {...},
        111: {...},
        112: {...},
        113: {...},
        114: {...},
        115: {...},
        116: {...},
        117: {...},
        118: {...},
        119: {...},
        120: {...},
        121: {...},
        122: {...},
        123: {...},
        124: {...},
        125: {...},
        126: {...},
        127: {...},
        128: {...},
        129: {...},
        130: {...},
        131: {...},
        132: {...},
        133: {...},
        134: {...},
        135: {...}
      }
    }
  },
  inspect: {
    pending: {},
    queued: {
      0x7ee9e416fdb371a9190dfa8fdf7361e66ada7e12: {
        110: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        111: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        112: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        113: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        114: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        115: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        116: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        117: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        118: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        119: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        120: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        121: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        122: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        123: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        124: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        125: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        126: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        127: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        128: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        129: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        130: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        131: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        132: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        133: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        134: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas",
        135: "0x8e195493424d416300f8b61b01145956345cd914: 0 wei + 4000000 × 18000000000 gas"
      }
    }
  },
  status: {
    pending: 0,
    queued: 26
  },
  getContent: function(callback),
  getInspect: function(callback),
  getStatus: function(callback)
}

that's the function in my contract which gets called:

/**
 * save sensor data
 * @param   data    sensor data timestamp;value
 * @return  bool
 */ 
function saveSensorData(bytes32[] data) returns (bool success)
{
    if(sensors[msg.sender].exists)
    {
        for(uint i = 0; i < data.length; i++) {
            sensors[msg.sender].sensorData.push(data[i]);
        }

        return true;
    }

    return false;
}