It's memory. You should reduce the memory multiplier since increasing the FSB will also increase memory speed. With the memory underclocked, get the CPU stable, then go back to figuring out the memory. Generally tho, if you increase the latencies it also might work. For instance, my RAM is rated at 1600mhz, 8-8-8-24, but I have it up at 1750 9-9-7-24 (althout that 7 doesn't seem to help with the latency in Everest benchmark). If you don't want to increase the latency and keep a higher speed, then it's just not enough voltage. The point of Memtest is to not test the CPU so it's pretty much gotta be a memory issue. Although, I'm not entirely sure which memtest you're using, I use MemTest86+ and as far as I know there's no selection of which test to run - regardless, you probably should just let it run all tests.