I missed that tidbit about the 2/4 twist. That might narrow your bug-hunt and reflow/patch wire project down to something on the order of the CAS/RAS lines of that particular memory bankage. It might only be the SIMM slot(s) that are the problem, I wish I had my Classic/Memory boards handy, they've been MIA for a while.

Where is the necessary wedging pressure applied to the MemCard?