Very helpful discussion, and lead on the Gauge Utilities, which I have used for many years but without knowing this particular virtue. Gauge's Ramometer identified a problem within seconds, and was able to find at least 50 more before I gave up counting and quit the test.
Given that the error is a gross one rather than the kind of subtle one that would take all night to find, I think I will try a quick swap of a spare 32MB chip for the six or so I have in there, one by one, running the utility each time. The RAM Sandwich looks a good deal more substantial, but less inviting.