PB G4 lower memory slot problem (again)

[mmsfo]

I have two 15" PB G4 1.67 Mhz bought at the same time in July 2005. Both have been identically updated since. Both stopped "chiming" upon startup after the OSX 10.4.7 update. I worried about this but failed to figure it out, and both machines seemed to run OK. Did the 10.4.8 update ... still no chime. Yesterday I discovered that both machines are no longer recognizing the 512MB memory stick in the lower memory slot, only the 1 GB stick in the upper slot. When I remove the lower slot stick completely, both machines boot with "chime". I can make both machines boot with a memory stick in the lower slot by resetting everything (PRAM, etc), but problem recurs if I try to load the upper slot with RAM. All of this sounds like the famous lower slot memory problem, EXCEPT that my two PBs do not fall within the "serial number range" of the Apple Slot Repair Program. Moreover, since it is *possible* to make the lower slot work, it seems clear to me that the issue is software/OS related and not hardware failure. My question : have others encountered this issue with the 10.4.7 and later updates? Thanks for listening.