RAM problem on P4B533

I just got a P4 2.66b for my Asus P4B533 and out of the box it goes up to
3.5Ghz (50deg C full load, default voltage, woohooh, can't complain .
I did the pin#6 trick to have a 3:4 ram multiplier above 133MHz FSB so I can
crank the ram up to 400mhz plus. I have two Samsung 400mhz dimms and if I
only put ONE in I can push the ram over 430mhz (860ddr), but if I put both
dimms I can't even run 400mhz stable so I have to drop the ram down to
325mhz. Aaaaargh. Anyone solved a similar problem? BTW, I chose all the
slowest ram settings.

TIA,
Muntz