Ya, probably firmware corrupting. 70% for sure. If not, it'll be some hardware problem.

Try these few steps to run a simple diagnosis:
1. Try using mp3 transferred from other phones, which can play normally. If can't run as usual, file corrupted; if can, phone or card is having prob.
2. Remove your card and start it. Do like what you did and see if the problem still exist. If yes, card is ok; if no, format card and see how.
3. Bring it to the nearest Nokia Service Centre (NSC), claim for warranty if possible.