Pc restarting frequently

And, just how did you verify the power supply is 'OK'? By looking at it, or did you use electrical devices to measure the input/output voltage? And, the RAM? How did you check that?

Restarting frequently is usually a symptom of heat, somewhere. How are the fans? Are they spinning to capacity? Are they coated with dust/smoke grime? How is the cooling tower/fan on the processor? Did you dislodge it from the processor when you tested the processor on another MB? Did you use new CPU 'goop' when you put the fan/cooling tower back on the processor?

And, last but not least, how about telling us what the specs are. What MB? What processor? How much RAM and in what kind of configuration? What wattage of power supply? How many fans? Can the computer breath where it's sitting, or did you block the air flow or trap it in a cupboard?

Not enough info...what type of board?? any post beeps to accompany this??? any error messages?? what type of O.S.???? What was the PC doing before the problem?? Was something installed, rebooted, error before this started happening??? Is the motherboard equipped with an overtemp control (sometimes the most common is a motherboard with an overtemp safety feature for the CPU and the CPU isn't cooling properly, e.g. reinstalled heatsink with no compond, heatsink came loose, etc...) the overheat protection will reach its threashold within seconds and shutdown the board (PC).
For proper troubleshooting though more info is needed, the more the better to isolate the problem. Remember we are not sitting in front of your problem, so any info that you can provide will help narrow down the problem.
Have fun!!

as already stated, check fans ect.

try testing temperature using a thermometer when computer is on.

try looking for bulging capacitors

if you have overclocked the computer, restore settings back to default in BIOS.

let us know how you go!

To see if anything is being shown there if the OS is constantly falling over.

Failing that and if you are running Windows you can try System File Checker.

But you really need to provide the details here as to what you have and how you are using it before we can help you out.

Col