^^ Shogun, to test things, try setting the game to absolutely minimal settings. If it still jumps around, you have some CPU throttling issue. Minimum settings do almost nothing in terms of lowering CPU load (like less than 1%), but will dramatically lower GPU load.
If it runs smooth with minimal settings, then you can try updating drivers + different combinations of settings to try to find a happy medium. For lower-end GPUs like that 1GB LE card, I generally like to go with :
Medium or high textures (depending on game, 'high' sometimes needs 2GB+ these days)
Low shadows
AA off
HBAO disabled
It will still look pretty good, as the most noticeable thing in graphics is the textures. AA is nice, but in motion it's less noticeable to have some jaggies, so the huge performance penalty isn't worth it if you're talking AA off = 55FPS, and MSAA on = 25FPS (which is pretty average with a 1GB card).







