I'm beginning to think something is up with this combo. At least with older Nvidia cards. I had a problem with my DS3L, 8400GS, problem with the MSI system, 9400GT and now the same with my GA-955GCMX-S2. I didn't have any major problem with the DC7800 and that was using onboard video to install and then using an 8400GS with DDR3 RAM. It leads me to believe that is the problem as I'm sure the older cards don't work as well with the 64 bit only OS.
Is anyone else having problems with these older cards using Mountain Lion?
From the DS3L system of mosslack...