Starting a few months back, whenever I would play any games (any settings, any resolution, Starcraft, Counter Strike, MW2, whatever), after a minute or two I start getting some really bad lag. I'm dual-booting with Windows XP, and I don't get any problems there, but obviously it's a big hassle to have to reboot whenever I want to switch tasks. I recently got a new graphics card to replace the integrated graphics that came with this desktop, and I was hoping the problem would go away, but it didn't.
The lag is also kind of weird. As I said, the games run fine at full 60+ FPS or something for a minute or two, but then decrease to around 20. After that, it'll be like, 20 smooth frames, then freeze, for half a second, another 20 frames, then freeze for another half second, etc. (it's not just like a consistently low framerate).
The lag happens with any game, old or new, online, offline. Windows 7 itself, Firefox, GIMP, whatever, all work fine, it's just games that lag (Starcraft/Warcraft also lag in windowed mode, so I don't think it's some weird full screen thing). It's also worth noting that the audio doesn't skip, only the video, and that if I Alt-Tab out of the games that the system runs fine, but going back into the game, the lag is still there.
My video card drivers are the latest, and my core temp is around 52 C. Can anyone think of something I'm missing?
Specs
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HP Pavilion a6530f, 2 years old
Windows 7 x64 Ultimate
AMD Phenom X3 Triple-Core @ 2.1 GHz
ATI Radeon HD 5770
4GB RAM, 640 GB HDD
500W Thermaltake PSU