The problem definately sounds hardware related to me as well. It certainly could be the AGP (GARP) drivers, or it could be a memory problem. I've personally sworn off VIA chipsets because they have a propensity to be so touchy about memory (I've ended up tossing 2 different VIA-based motherboards after fighting with memory-related stability problems for months). If that's a VIA board, I agree with Alex B and would say that "Auto" might not be the best choice.
Stress Prime runs essentially the same tests as Prime95, but uses a nice GUI. Prime95 is great for testing the CPU and North/Southbridge on a motherboard, and "by association" helps to test the memory and AGP bus as well, but it does not explicity do so. I would definately suggest running Memtest86 overnight on the computer and see if it lives through a few passes. If not, then start Googling for RAM stability issues with your board.
In addition to playing with the Timing settings, you may also end up needing to bump up the VCORE for the RAM setting a bit to get the sticks to run stable (though I too would do a search before I started messing around with those settings).