I'm dealing with an Optiplex GX280 that was randomly freezing and then randomly shutting down. After a little bit it stopped turning on altogether and the only things that would happen were that I would get a yellow light and I could here the CPU fan speeding up to it's max speed. After I reset CMOS it started up normally again but after a bit started randomly freezing again. I'm rather puzzled. Could it be the PSU? and if so why would resetting CMOS make it start up again?

Other info: Apparently there is a common problem with capacitors on motherboards of the GX280 (see this thread) but if that were the case resetting CMOS wouldn't have fixed it, so I'm leaning toward the PSU, but then why would resetting CMOS fix it and why would fan speed up?

Any help is apreciated, thanks.

I would think that it is most likely the PSU as you suspect. I am actually working on an Optiplex GX150 with the same problem right now. I found documentation on the Dell support website that stated if the power light on the front of the case was a solid amber color that it means there is an internal power problem. Try using another PSU to see if it continues to happen. As for resetting the CMOS haveing an effect on the machine; I don't know computers can be funny things, it could just be a coincidence and the PSU is having an intermittent issue (which are the most difficult issues to diagnose).

Did u unhook everthing (not the power supply ) and then try it ?