I agree with T5BRICK with running with AHT on your unit...as long you still have Snow Leopard installed; when powering on the unit, hold down on the "D" key to enter AHT mode to run the tests.
Are you able to boot to an external drive with a pristine imaged copy of OSX? If you are getting the same issues, it will be leading towards a "intermittent" problem with the logic board.