Hi.
I had the same problem. Asus A7V133, VIA chipset and GForce 2 MX400. Huge lockups when gaming. The solution was opening up the case and directing a fan onto the motherboard. Just like Crusade_Asus wrote previously. No more AMD nor VIA for me after all this suffering. It took me almost a year to find out the solution because, my first board was a SIS 305 AGP. In the beginning, I blamed it for the problem.