go download a freeware spyware search program (spybot search and destroy) and run it. Afterwards take a look at your running programs (alt+ctrl+delete).
http://www.duggmirror.com/software/Tweak_XP_for_gaming/
Have you installed anything new in the past couple days? I'm going to Daytona for a weeks vacation so I wont be able to reply, but whenever I have issues like this those are the first things I check.
If that is clean, then check hardware. Hardware is usually not at fault when FPS jump around like that, it's usually software related.
Good luck,
Joe