i suppose you could have a bad stick of RAM and just need to find it, or it could be something worse like a weak or in the way out power supply.

make sure all of the memory is identical ( like 5-5-5-15 ) mixing and matching is NEVER good.
try removing half the memory and run for a while see if it still has the problem.
then try again with the other half of the memory installed and see if it still has the problem.
if it's just a single bad stick of memory hopefully you can find it this way.