No subject

metal of the board connector does not match the metal of the contact
area on your SIMMs. I saw similar problems before with self-built SDRAM
SIMMs. The only way to get it working reliably was to use a board
connector with gold plated contacts and to use a SIMM with a gold plated
PCB. I am not a chemist or physicist but I think else the two different
materials after a while build up some strange alloy / oxide that ruins
the contact.

What's the plating of your board connector and what's the plating of the
SIMM contact area?

Regards
Mark