HD4870 lagging on source games

Well i've been playing some dod:s and portal and I keep getting this kind of "pulsing lag" the fps keeps dropping from 270 to about 15-30 fps (on portal) and is a bit worse on DoD:S i tried hl:ep1 and the same thing's happening so I think its the source engine making it lag. I dont think the 4870 would lag on these games because I used to have a 9600 and it would run perfect. my speeds are 819 (gpu) @ 1022 (memory).
I dont think its a bottleneck either because a friends 9800gtx worked fine on my build.

E7200 @ 3.2mhz l 4gb of mushikin ram (ddr2 @ 900) l 500w psu l Sapphire HD4870
would anyone care to help?
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More about hd4870 lagging source games
  1. If it's been in the past couple of days it was steam maintenance. It was doing it to me too. It should be resolved now. If not then that sounds like video drivers or something. Turn on your vertical sync though, there is no reason to have 200 fps when your monitors refresh rate is 60 hz.
  2. to be honest it was actually doing it right now
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