I used to use a PSU that came w/ the case. it was a 500w P.O.S.
it would cause fluctuations in my fps, and sometimes BSOD my computer during games.
It could be the power supply isn't sending enough stable juice down the 12v rail.
The PSU is almost always the most under appreciated part of a build.
I like BFGs offerings on PSUs, and i use them for most of my builds.
Find one that is @ least 80+ Certified (energy efficiency)
**before you swap out the PSU (which you should do anyway, i've never been a fan of 'generic' power supplies, to unreliable and energy ineffective) make sure there are no background programs that are conflicting. Close every background program that you have. I've seen Windows Media Center downloading 'program updates' effect FPS too