PC locks w/ added memory - what next?

Asrock mb with a Pentium 2.8 Northwood 800 Mhz. Radeon 9800Pro. Had 512 mb ddr
PC3200 with no probs over last couple of years. Put in another 512 and get lots
of locking up (using any 2 of the 3 slots). Memtest showed nothing wrong running
with both sticks, and a single 512 of either stick installed exhibited no
problems either.

Borrowed two matching 512's (Geil) thinking there may be some timing issue but
same problem.

At this point: can it be anything other than a flaky mamaboard? (ps? weird video
card / 1 Gb memory interaction? or ?)

Thanks -

zach
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