On a couple of other points from the original post: stop bath transition time is not really significant at development times around 10 minutes or so. Depending a bit on the tank, it usually takes about 15 seconds to dump the developer and pour in the stop. The drain time does not really count as the emulsion is saturated at that point. And you will be come consistent with a bit of practice.
If your thermometer is really off, or your tank temperature is very different than the solution temperature, your effective development may not be what you think. It is unusual to have a tank significantly warmer than the solution, and the solution temperature is going to dominate the heat balance. Try to work in a room close to your developing temperature; mix up a bucket of water at the process temperature you need; ideally work to the same process temperature - I usually use 21 deg. C. The benefit of working to the same temperature is that even if your thermometer is not accurate, it should be consistent at a given reading. Make sure you give the thermometer time to settle, and if it is a dial model, be sure to read it from directly overhead. Thermometers, unless carefully calibrated tend to be less accurate the further you get from their calibration point - they are not always linear.
Temperature - like time - need not be accurate, but it has to be consistent. There are enough variables in photography not to lock down those you can.
It may be that you are under exposing and over developing - I know I did when I started out - or it may just be the scanning limits.