The problem can be caused by the suprasolicitation. If you don't turn it off regullary and you use it 24\24 it can be broken. Another cause can be the very full memory.
The flash memory is probably becoming corrupted on your player. I'm sorry to say, but GPX players are at the bottom of the barrel as far as quality. Upgrade to a sandisk sansa (inexpensive) or better to avoid this problem.