From a cold start my desktop boots up and runs ok. However, after warming up it starts hanging and even fails to boot up if I persist with restarts. What could be the cause? NB. The power supply rating is my first suspect but the rating is not clearly labelled. Problem started when previous power supply failed after running problem free for years. When I know how to safely remove it, I'll take a closer look at the rating.

dont go trying to look inside it unless your well clued up on electrics, you can get a nasty shock (if your lucky, if your not lucky we will be sending flowers, and that goes for psu that have been unplugged for months, treat them with respect). good news is from what your saying its more likely its overheating, try blowing out all the dust and crap from the HSF that cools the cpu, and it would be well worth taking it off cleaning the surfaces of the thermal paste and renewing it (artic silver is good brand) you only need to remove HSF you dont need to take CPU out its too easily damaged

I'm agree with Popeye, looks like overheating do what he is telling you; and let us know the results.

Thanks Popeye67 & mtm. Dust is not the issue. The desktop was cleaned out by the repair facility that replaced the power supply. I was not too happy with their trouble shooting [hence need to be my own IT Guru]. Suspecting a component either on power supply or motherboard that fails after the computer warms up. To rule out power supply, I want to try another power supply unit. One thing I am not too sure of is how much force is required to disconnect from motherboard.

Advanced SystemCare provides a diagnostic profile that gurus can analyze and then come up with the way forward. Are you [Popeye67 & mtm] familiar with this? If so, I include the data as an attachment.

Lot of people on here swear by it, problem is if you do a fresh install and then run it you will get hundreds of so called errors and problems it is offering to fix and there anything to fix. if you can get hold of a decent power supply it will rule it out, they can be pretty stiff to remove from the atx 20/24pin connector see if you can support the board when removing it, whats the make model or power rating of the psu if you can find it and the basic specs of computer anything above 300 watts should be fine but not all psu are created eqaul and cheap brands can be very poor performance wise, there are cheap power supply teasters avilable for few £/$ on ebay,amazon

psu tester | eBay

Thanks Popeye67. Different power supply [550W compared with 350W] managed to start the PC but an unsteady hum compelled me to turn off and inspect the inside. With one PC cover off, I noted the CPU fan was unstable. With power off I compared the two fans and the CPU fan was definitely stiff. I serviced it [with WD40] and it started operating freely. I was even able to install some updates and then shut the PC down. Before restarting, I tried the old power supply with the serviced fan but both fans turned a little while and the PC powered off. I got the same response with the 550W power supply. What can be the problem now and how can I sort this out? The good news is that you don't have to send flowers yet.

I would say its either a bad conection somewhere causing an intrermittent fault or the cpu fan was on it way out and its finally quit and needs replacing but unless its failing to run at all its more likely to be bad connection, computer will shut down pretty quick if fan isnt running but if it is it would run for a few minutes or more

your perfectly safe in the pc case (your more likely to damge it, than it you), just avoid opening up the power supply or be very carefull capacitors can get pretty big on some and hold a charge for months

As has already been mentioned...the most common cause of sudden shut down is overheating. Resolving a fan issue may not fix it. I would suggest removing the fan and the heatsink. (Be sure the power is not connected.) very carefully clean off the top of the processor using a lint free, clean rag and a tiny amount of alcohol on the rag. Clean the old heat compound off of the CPU, and off the bottom of the heat sink. Re-apply a new, even layer of thermal compund. Reassemble everything. Use the computer. If it still shuts down after a few minutes it could be that the processor has experienced thermal damage that cannot be reversed. The CPU has been weakened and the thermal threshold has been lowered too much. A new processor and a good CPU fan may be the only fix.

Thanks for all contributions. It turned out that the power supply had failed. I had identified that the power supply was inadequate but now I appreciate that full power has to be available before the CPU can operate. One more question: To confirm that a fan [CPU or cooler fan] is free, I would give it a spin. However, I heard from an IT show on radio that when blowing dust from computer fans have to anchored to prevent regeneration which could harm the PC? Can I have some clarity on this?

regeneraton ? is it going to come alive and attack you. you should hold the fan still but the reason is if you dont the bearings can get damaged nothing to do with creating an electrical current