On some system architectures the problem could be that different capacity memory chips are mixed together. Someone might have placed 1-Mbyte SIMMs in the same bank with 4-Mbyte SIMMs. If this is so, rearrange the memory chips. Make sure to put higher-capacity SIMMs in the first bank(s), and lower-capacity SIMMs in the remaining bank(s); never mix different capacity SIMMs in the same bank.
The problem could also be that cache memory on the motherboard has gone bad and needs replacement. If main memory is installed correctly, try swapping the motherboard.