RAM sticks won't overclock together

[kevral]

Having a problem I can't figure:

Pretty much any overclock makes my system Prime95-unstable. So I figure perhaps it's my RAM, and I decide to change ONLY the fsb/dram ratio, and see what happens.

So I leave fsb at 166 and set fsb/dram to 5/6, giving me 400 mHz RAM speed. Boom. Unstable immediately.

So I pry out one of the two RAM sticks and retry. Stable for 20 minutes.
Switch the stick for the other one. Stable for 20+ minutes.

So it seems that either stick of RAM will work fine on its own, but they won't overclock together!

What do I do?

[Matthias99]

Did you try lowering the timings? Some memory controllers can't deal with dual-channel mode at high speeds and tight timings. You also didn't say what kind of MB and RAM you have, which might help. :)

[kevral]

Knew I had forgotten something important.....

Running an Abit NF7-S v2.0,
Athlon XP2800+,
2x 512 Twinmos pc3200 w/ Winbond....

So it might be too tight RAM timings? Running at default "optimal".
What should I try?