Steam Gaming Problem

[TMD]

Been playing a number of games on steam the last few days...and for the first 15 mins or so everything is good...however after that at regular 4 min intervals for about 30 seconds the whole game starts lagging as though there is another process taking cpu power. Ive got two screens so i tried running task manager on the other screen while gaming...there was no difference in the number of processes or cpu usage. Also everytime it happens on the second screen a black window about 500X500 pixels pops up for a fraction of a second...but its so quick i can't tell what...anyone any ideas...its really frustrating!

[danroyle]

vista or xp? if vista run it in compatibility mode runs way better

[TMD]

Its running on XP