Huh, well I'm having very similar issues and also on a Gigabyte motherboard. I'm running a GA-K8U-939 with an opteron 165 processor and an nvidia AGP graphics card. I occasionally get the BSOD with the MACHINE_CHECK_ERROR message. I've had this setup for about 6 weeks and have received the error only twice. I'll try downgrading the graphics drivers and will let you know if that fixes things for me as well. May take me a while to report back since I get the BSOD so infrequently.