Memory not functioning at the specified access rate as required by system board. If the system specification calls for 80 ns access rate, Windows NT most likely fails if memory is accessing at a slower rate such as 90 ns. Even though the chips may be marked as 80 ns, in testing, some fail to meet this access rate. Quite often memory chips run at a slower speed when they reach operating temperature. A high quality SIMM tester can cycle the chips through various voltage and heat cycles, so this is fairly easy to see.
Memory meets system specs, but speeds are different between SIMMs.
The average access rate may be 70 ns on one SIMM module while the next is running at 60 ns. We have found SIMMs stamped at the factory to be rated at a 70 ns average access rate to actually be running as fast as 50 ns. Although the SIMMs are obviously well under the system required access specification, the difference of 10 ns or more between them can often cause problems on some systems. An interesting note here is that you can move these to a different system board which is using a different BIOS and chip set, and it may not have any memory problems. The downside to increasing the number of wait states is a slower system.