Oven – Using the leftover oven heat

It sounds like the oven needs desperately to be looked at. Please have someone check it out before a fire starts!!!!

The thermometer placement isn't a huge concern. It should be near the middle for general use (top to bottom) and off to the side. Or if you prefer you can just hang the thing in the middle. As long as you can always safely see it it should be fine. If your coils are on the side and not top and bottom put it in the center (where the window would normally be)

Really though I can't stress it enough. If your setting it to 350 and sometimes it is temperature x and sometimes it is temperature x + 100 then you really need to have it looked at before it gets worse.

I think you may reach the point of diminishing returns if you try and fix your oven because there's lots that could be going wrong. It could be that your oven design is just bad, in which case you can't fix it anyway. It could be a faulty temperature sensor, or a bad gas valve, or a faulty burner. Your gas supply pressure could be inconsistent. By the time you replace all these you'll have spent more than a new oven would cost.

I'd first clean it really well. Get all the dust and gunk out and see if that helps. Then you should see if you can find a pro who can have a look at it, test the gas supply, see if there's any quick wins.

One suggestion I'd make would be to post this on the diy stack exchange forum, the home improvement site. As this is more a question about mechanics and repair you're likely to get a better answer there.