What are you looking for it to do, lou? System Mechanic Pro is a good one, but these programs tend to have a 'mind' of their own and can deem things harmful that aren't... give me more info to work with.

"diagnostic" covers one heck of a wide field.....you might need to be a little more specific...

Sounds like it could possibly be due to overheating.  You might want to open it up and look to be sure that all your cooling fans are running, including the CPU fan.

I assume you are talking Desktop with the factory installed original PSU.

What brand and model of computer, how old is it, and what peripheral devices do you have hooked up to it (including usb ports in use)?

Have you added extra drives or graphics cards?

Does the machine just shut down or does it reboot itself?

Fortunately, PSU's are neither expensive or hard to replace, more so  than a mobo, which could also be the problem.

Do you have  adequate ventilation of the PSU? Many have an intake fan as well as an output fan. Make sure both are clean and running. Speedfan does not monitor the PSU temps, does the discharge air seem overly warm to you?

I suggest unplugging all your external stuff and see if the problem goes away or occurs less often to try to pinpoint the cause as internal, external, or an overloaded PSU.

If you wait awhile before restarting,(let it cool down) does it take longer to shut down on it's own?

 I have a total of 7 fans on my rig and 3 cats in the house. Hairballs!!! Needless to say, I have to clean my fans frequently. Hopefully, it was just an airflow issue on your machine.

Turns out you're not the only one with problems at the moment, lou...

lol am not making any comments  

As a fellow zookeeper, I have found it very useful to put filter material over the air intake sections of the box.  Nothing too heavy or tight, don't want to choke it off, and nothing that tends to drop strands/filaments.  Something along the lines of low-end furnace filter material if it's not too fuzzy.

I have a Cooler Master Centurion5 , the entire front is air intake, with filters. The added side fan is also filtered. It sits in a desk, yet it still picks up crud at an alarming rate. I love my cats and my computer, cleaning it is a minor chore to me.

Here is a Dell that came to me for repair. No such luck, it was DOA. Any idea why?

well folks....good news, the pc running a treat now its been severely hoovered!!  nice cool air coming out and no shut downs....YIPPEEEEEEEE

He must.... or there wouldn't bee so much dust.