I was having the same symptoms with a Hummingbird POS.
I loosened the screws for the cover a little, and it would stay on for ever. It was overheating!!!!!!!

So I trashed the HB and got a Lowrance Globalmap 2000. No more problems.

I would suggest loosen the screws a little and see if its a overheating problem.
I really dont think that is a problem with a Garmin.But, its worth a try.