check all your settings, make sure you are running in 32-bit. It sounds to me like either the obvious (malware), RAM, or a GPU problem..update your GFX card drivers...give me some info on your pc...CPU socket, speed, fsb speed, ram (how much?), your gfx card mfgr and model...maybe your PC is just gettong bogged down...try leaving it alone for a half hour or so..if nothing still, then i'm pretty sure it's prolly a piece of malware eating your RAM...(unless you have other GFX-intensive software running at the same time..then It could be video-ram or not a fast enough GPU)