Ive been having similar problems, fans kicking in at every 20-30 seconds, causing that dreaded unwanted delay, just when it counts!!

I have a AMD 64 bit 4600 (Duel core) (that I still think, isnt properly set up!!) I have an ati 800 256 and 2 gig of RAM. Was still having this issue whether in 64 player or 32. I took my PC apart, pulled out the vid card, sound blaster card, removed all the sticks of RAM, cleaned with anti static wipes, and "dusted Off" the entire PC.

Well, that issue with that sudden lag has disappeared, i thought it may have been something major, but it wasnt!! It was DUST!!! Its running wonderfully right now. get the odd spike, but thats an Internet Issue , not mine!

Old saying goes, sometimes, when in doubt look for the easiest fix. Its cleared mine up! Hopefully yours!