A while back I changed my memory to 2 sets of mushkin 2GBs. I had one bad stick, so I returned that set to Newegg. No problem after. They are not a great overclocker, but, they work at 420 xDD2. I used the Microsoft Dos program to find the bad stick. Memtest is also good. Maybe you need to test one at a time. I had error in a very short time. For me, no long test was needed.