Memory stability

Right now I am having some stability issues with my memory. I have been able to run orthos with my processor clocked for ~9 hours no issues. Trying to run memtest is another issue. It seems to fail after ~7minutes of the test.

DRAM Frequency: 800Mhz (1:1)
Mem Voltage: 2.05v
NB Vcore: 1.55v
Timings: 5-5-5-15

Really not sure what to do at this point. I have tried tightening up the timings, and changing the voltages for the memory modules as well as the north bridge. Anyone know what I should try to do to get some stability? The rest of my specs are in my sig, if you need info about settings and such please let me know.

Thanks.

Orthos "small FTTs" only stresses CPU.
"Large FTTs" and Memtest HCi/86+ are much more accurate for testing memory.

There's not much you can do except:
- lower frequency
- higher latencies
- add ram cooling
- increase voltage
- buy new memory

What brand of memory? Any performance DDR2 should be safe up to 2.1v.

OCZ DDR2 PC2-6400 Gold Dual Channel (OCZ28002048ELDCGE-K)
I have 4x1gb sticks.

I am trying to do this currently with only 2gb.