I guess try what BT and AA250 say, I just find it weird that you said you keep configuring in plugins and things go good for awhile and then go back to bad. That to me sounds like a system problem. I don't know, could be anything, Try what they say and see if thats helps. I have never had any problems with MP3's other than some of them not playing cause the file was too large.
Edit: One more suggestion. Try updating your sound card drivers. See if this helps.