If the power supply has been changed 3 times... why? Was it verified bad when it was changed before? I would do a voltage test and be sure your getting the correct voltages on the low voltage supply. Should be +5, +12, -12 and a good ground. I would say if the others were actually failing that fast then its a good chance who ever worked on it did not find the problem that was causing them to fail. Your new supply might be bad now again as a result. Three supplies going bad that fast... I would start looking for another issue causing it.