i just got an hp pavilion elite h8-1039 with a 6850 in it, and since christmas eve have been playing games like sniper ghost warrior on max with no problems till today. i booted up the pc and started playing Sniper GW and it lagged a lot when it hadnt previously, granted i no the psu is low but still..till now ive never had a problem even after 3 hours of gaming, what could it be?

Could be:

1) heat
2) virus program checking every thing that gets started
3) some other new program

Use msconfig to disable most starting apps and see how that works out.
And use HWMonitor to find your maximum temps while gaming.

And seeing as it's under warranty, give HP a call.

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#1: Phenom II C2 955BE @ 3.6, Antec Kuhler 920, Gigabyte GA-MA790FX-DS5, Asus 5870/V2, ASUS Xonar D2X, 4 x 1gig OCZ2N1066SR2GK @800, Antec Truepower New 750, Arc Midi, Win8pro x64 upgrade
#2: Athlon 64 X2 4200+, AGP 1950pro, Audigy 2, Asus A8V-D, 2gig ram,  Enermax 465P, Antec900. Now on Win8pro cat 10.2