P2511 - continual rebooting

[neilhughes]

I have 2 x P2511 Esprimos that have started rebooting themselves every few minutes.
Both bought at the same time about 2 years ago

Have replaced both ram chips and power supply and they still do it. However if I only leave 1mb of ram in the machine it is fine. As soon as I put the second chip in, it goes back to rebooting itsself. Have tried they in different building to make sure there is not a supply problem. I have downloaded and run the Fujitsu test programs and everything checks out ok.
Any ideas - please