Computer restarts does aTimed System Memory Check

Hi I Have Windows 2000 On an Athlon XP 1900 with an Nforce 6373 motherboard. I have had problems using two 512mb of 2.5 cas latecy ram that runs at 266mhz. I boot up with 256mb of cas latency ram and than than in the bios the cas latency form autodetect to 2.5 and the ram works. I ahve tried the computer and it works good with my cablemodem and stuff. Heres the problem I set up this computer at a friends house who ahs DSl. It works fine but after 1 to 2 hours the computer restarts and he said it does a timed system memory check. What is causing this I have never seen this message before