Help! I'm trapped (type 8) (4.1bsd question)

[utzoo!decvax!harpo!floyd!vax135!cornell!uw-beaver!jim]

My usually-faithful Vax 11/750, Beaver, has recently been crashing
about once a day with trap type 8.  They never happen in the same place
twice.  About the same time this started, we found some bad memory,
which we replaced, but we still get the traps.  All memory passes
disgnostics, as does the processor.  I haven't called DEC field service
yet, but if the machine passes diagnostics, I don't know what they
could possibly do for me.  Has anyone seen this before, and can you
help?