GE Spacemaker microwave

My microwave would run for a few seconds; on the panel a light would flash dimly, and it would cut off, asking for you to reset the clock. Reset the clock and it would do the same thing regardless of whether you used the expess keys or a specific button (ei, Beverage, Popcorn, etc.) We replaced the control panel assembly, but it does the same thing (not quite so bad, maybe.) WHAT IS WRONG?? and where can we get a repair manual showing parts and a diagram? Thanks