vaxstation ii machine check & diags efficacy

It's a VAXstation II/GPX. I'm trying to install VMS 5.5 (from CD if it matters; there is a CMD CQD-200 or -220 as the only storage controller) and routinely getting machine check exceptions. Standalone backup always boots. Sometimes I get an exception the moment I hit <enter> on a restore; sometimes in the middle of the restore, sometimes it runs to successful completion. VMS never boots. Sometimes I get as far as the date prompt (I took out the battery), sometimes it gets a little farther, frequently it's before anything appears on the console at all. The exceptions booting VMS occur the same if I attach another disk with VMS 5.5 already installed; if I replace the KA630 and M7609 with a KA655 and M7622, it all seems to run fine. The KA630 power-on diagnostics all pass, but... it smells like a memory fault to me. What are the odds there could be a memory fault that isn't caught by the diags? ok bear.