i have a similar problem.
I don't use the pc software "music manager" or pc suite or whatever (I DO have the drivers and stuff to make the PC recognize the Phone and I know i have to constantly "Update Library"). I select "Printing and Media" mode on the phone and use Windows Explorer to drag and drop .mp3 files.
In my case, the first problem was it would only play one file. I would try to play some other song, it said "Please Wait" for 30 seconds and started to play the same song again.
Couple of days after that, it started take really long to "Start Music Player" and also to "Update Library". And it was showing (and not playing) songs with wrong artists and album names, like:
"Do It Again [a song by Beach Boys]
Amorica [an album by Black Crowes]
Daft Punk"
Now i have to listen to my files trough the phone explorer ("Gallery") , which means the lights will be on all the time when music is playing, I won't get acces to any other phone function while music is playing AND i can only listen to one song at a time, I'm not able to just make it play a whole album or folder and leave it in my pocket....
That's driving me crazy. I bought the phone 3 months ago...