Next time lower the voltage to where they are stable at the manufacture timings of 4-4-4-12. JEDEC minimum is 1.8v so I would start their, run memtest no errors? great no more playing is needed. If you get even one, bump it up to 1.9v, and keep repeating until it is stable. At 2.1 - 2.2v you really should have a fixed fan on them to keep them cool anyway. Also if your running 4x 1GB sticks, I assume you bumped up the northbridge voltage too right (usually needs to be done by +.01 or +.02)?