[Joe_]

I know I keep saying your PC/GPU is not overheating but.. eh what can I say.

My old PC, the BSODer, always got this when it overheated, but I really don't think that's your problem..
Seeing as I get this too, and I also get the flashy textures.

Have your updated DirectX?
Changing Anti-Aliasing
Changing DD?
Also try rebuilding your GTA3.img archive seeing as it looks like it's picking the wrong textures? :S

Joe_