[CORed]

The motherboard LED is not definitive in determining whether the problem is the motherboard or the power supply. I had a power supply fail on my Dell, and the motherboard LED was lit. I didn't have the intermittent type problems that you did. I just turned off the computer one day and it wouldn't turn on again. I looked through the Dell forums, and found that quite a few Dell's sold about the time I bought mine had had power supply problems. A lot of people had called Dell support, and been told that if the motherboard LED was lit they needed a new motherboards, and found that the new motherboard didn't solve the problem and then had to buy new power supplies. I replaced the power supply and have had no hardware problems since.