could be the battery. How old is the battery? NiMh or Li-Ion?
The battery it's LI-Polymer 3.7 V, I guess It's not the battery because it only do this thing with the same songs and when it's connected to the AC adaptor too.
I just started having the same problem. All of a sudden, my 2840 quit playing wma files. I'm not having any issues with mp3 files. I've tried updates, but haven't had any luck. Also my profiler quit working. I now have to profile from my pc