[Ethereum] Ethereum wallet keeps redownloading entire blockchain

I'm having serious problems with my wallet. I was running fine up until a couple of weeks ago, then when opening the wallet I was asked to update. I did, and ever since I've had nothing but trouble.

So firstly, I'm on Windows 10. I have 10mb download, and the data dir is on a mechanical HDD.
When I first updated to 0.8.3, the syncing was excrutiatingly slow. Maybe 100 blocks an hour, if that. Eventually I gave up and attempted to redownload the entire blockchain with Geth --fast. This got a lot further, but still wouldn't sync to the current block. I updated to 0.8.4 and finally managed to sync, but if I exited the software and opened it up even an hour later, it would take the best part of a day to sync back up.

Finally I got somewhere with using Geth separately before running the wallet, and after another three days of syncing, as long as I ran Geth --cache 1024 before starting the wallet, it was usable.

Except that at some point the other day I accidentally ran the wallet without running Geth first – and it started downloading from Block 0 again.
Even if I then exit and run Geth separately, I'm still stuck at block 0.
So I downloaded the entire blockchain again, and lo and behold the same thing is happening. I'm back to block zero after restarting the wallet.

So it seems that the various methods of running Geth aren't interchangable – even though they're the same version and looking at the same chaindata folder. That doesn't seem right to me – but it seems I'm stuck waiting for the wallet to download the blockchain all over again, assuming it can do so, without being able to use geth --fast, if I ever want to keep it stable.

This doesn't sound right to me, and it's going to cost a fortune in bandwidth!