GA-71XE & RAM problems

My older second machine has a Gigabyte GA-71XE and an Athlon 600 cpu. Recently the machine has developed an error in the memory system. I can get a single 256mb stick to work but if I put in more I get am error with Nortons diag. and some instabilty. I have tried 3 different 256mb sticks and all will work in any slot on their own but not if any other stick is in the other slots.. The story is the same if I use 128mb sticks - one stick is OK but 2 or 3 and it fails. It does not seem to matter if I use 100 or 133mHZ ram

Is this really a ram problem or is the motherboard faulty??

Thanks
Fred