Both modules mentioned are by Crucial. Thus far, the module that didn't function reliably in the ThinkPad R51 seems to have no problems in my Compaq NX6110. Of course I'm still tracking that module and have a couple of backup modules on order. The modules on order are not by Crucial.
I finally got back to this ThinkPad R51 stability problem and was able
to definitively assign blame to a defective (at least in this box)
memory module. Defective memory was suggested on list as a probable
cause, so thank you. I first used the "mem=1G" kernel boot parameter to
limit memory used to the lowest addressed 1GB part out of two. When that
configuration proved stable, I then swapped the memory module in the
expansion bay, assumed higher address, with one in another laptop that I
have. That configuration proved stable as well. So problem solved for
the ThinkPad R51.
