Each SODIMM worked fine alone, in either slot. The new SODIMM was the exact same spec as the old one, same size, same manufacturer - but different part number. The order mattered in the end. I started from scratch (=boot the CDROM and install) and everything was fine after reflashing the BIOS. The machine was just as fast as before, other than a little extra time to boot (memory check no doubt). Very strange problem.

BTW I was not unproud of this diagnosis.