Well got it running @ 350 GH/s stable, with chainminer autotuning. I beefed up the 12V and ground rails beneath the M-board and also now feeding power to the connectors on the other side of the m-board, but don't know how much that actually helped because i needed to do more card switching to get all cards show up.

There's still one card showing only 20GH/s, few chips are almost 0GH/s. That has has been running over 30GH/s before, so there's clearly something still not quite right. I haven't upped the voltage yet, maybe i'll try to see if overvolting the card or cards in the same bank would help.