Sounds like an excuse to go shopping for a new one if you ask me. I've had the stereo (non nav) in my '04 go wonky every so often as well. It will refuse to power on or off and all the buttons are non-responsive. I usually have to pull over and shut the car off for about 30 seconds and then everything's fine again. I can't seem to correlate it to any other factors.

Yours on the other hand, sounds like its heat-related.