I'm in a 25 man raiding guild in WoW, and for some reason, my PC hates 25 mans. I can run the game on Ultra almost all the time with 45-50+ FPS (only 25 in Dalaran, but that's expected). However, the second combat starts in a 25, my FPS goes to hell. In 25s I play on the lowest settings possible, and I still get 10-15 FPS maximum.

Some stuff to know that might help diagnose my issues...
I play on 1440x900 resolution, my PC has a 1.8Ghz dual core processor, 4gb 800Mhz DDR2 RAM, nVidia 9800GT video card, 700w PSU, 320GB 5200 RPM HDD, Windows Vista Home Premium 32bit. I play on DSL but I usually have 100-150 ping, so I don't think server lag is an issue.

I don't expect to be playing on Ultra GFX during huge 25 man raids, but I'd like to at least play at decent graphics, the game just looks like a garbage slideshow right now, with my low FPS on Lowest settings.

Thanks! =)
I make sure than nothing else is running, only things I have up during raids is WoW and Ventrillo, and EVGA Precision (my graphics card overclocking program). My gfx card isn't OC'd much though, only ~10% since more seemed to make my PC unstable.