LAG!

[=TFU= M-Coke]

Just upgraded to Win7 reinstalled all my games etc. including PR. Now previously on XP my system E8400, ATi 4850 with 2gb of ram would run PR fine with everything on high. With the upgrade to Win7 I also bought 2gb of extra ram of a mate, unfortunately one stick was busted so I currently I have 3gb of ram.
I running PR at 1680x1050 with everything on high and 4x AA. It runs fine for about 15-20 mins with frame rates at around 70, after though rates drop down to <20 and stay there no matter what seems to be happening on screen...

I tried deleting my BF2 profile, reinstalled PR and tried adjusting my page file. But it doesn't seem to help.