When I was an electronics techie in the 50s I carried a LOT of duplicate valves, especially for tuners. Some would work in some sets and not in others. Not assuming new = working is a fundamental part of fault-finding.
Another swap is clearly called for, but what if that doesn't do the trick? What could limit the system to looking at 1 strip?
I suppose there is the remotest chance the computer is zaping the memory . . . sometimes. Maybe this is what your supplier will suggest. 1 gig strips are as cheap as chips (erm, they are chips) because people swap them for 2 or more gig strips. Any forum member got some used 1 gigs to test it with?