to do a proper test on RAM you should run something like memtest
. so do like you did before and leave 1 memory stick in and run this for a few cycles for a thorough test.
the other thing that might cause the pc to randomly reboot is a faulty PSU. so can you get the hold of another PSU (with sufficient power) to test it?
did you ever get a bsod, that you could search online to isolate the problem?