Oh I hate that. You can set your phone to not do the auto brightness thing, but if it's overheating then the dark can also be the lcd getting too hot as well or the phone going into some sort of super low power stupid mode to try to cool off.
For the GPS, I used to have that problem with my boat GPS, Garmin had heat issues for a LONG time, not sure if they ever corrected them or not, BUT.. in the settings you can set it to show / do 2D instead of 3D on the graphics, which would significantly reduce the cpu use and help keep it cooler. Id imagine if you have a setting like that, it could help a lot. Also settings like turn off surrounding terrain etc, as in, do I really need to see the 'wavy grass icons' all lined up on the side of the road to know it's grass? type thing.
Aaron