This defently sounds like a software issue not hardware as you say it works fine with Sp2 only goes bad with sp3. Are you using driver from nvidia site or driver from gfx card maker eg HIS or asus ect. Are you adding sp3 after xp is installed or is it a sp3 version of xp with sp3 already included. IF not may be worth trying xp + sp3 included. Also are you running bare install no virus software running or any thing else just XP?
