Sounds like as if the virtual memory drops out, how much system memory do you have? Cos if it is more than a gig, i don't understand that. Also check all your background processes and applications seems as if some might be resource hogs, might even be a virus.
Also check your temperature inside your PC, sounds like their might not be enough cooling for some parts, as the performance drops as temp picks up.
HOpe this helps, a little bit. Good luck, Sherlock.
There are only Two things in life i regret doing... 1. Selling my Soul for Cash.... 2. Buying a Compaq.... I should have kept the Cash, huh.