Well my base unit is two years old and I've a EVGA Supernova 750W Fully Modular 80+ Gold PSU. Last week when I went to start the PC in the morning it came on for 1 sec with lighted fans etc, then dies, but the memory that is also lit up, stayed lit, but nothing else happened. When I tried to boot again it was ok. Then again this morning the same happened again, but it wouldn't start after trying quite a few times. So I unplugged the ATX connector from the mobo and a few other cables, tried again and the same. So I then tried to unplug all the cables from the PSU itself, plugged them in again, and now the PC boots again. I'm using it now, but have not tried to restart it again since I got it working earlier.  The PC has been freezing lately, probably 3-4 times a week, then having to hard reboot, Do you think the PSU is faulty? If so how long is the warranty?

Thanks for the reply. It's an X3, so a 10 year warranty. I'll be very, very disappointed if it is the PSU as I expect more than 2 years from a top end PSU. This is why I buy quality components. Also if it is the PSU it means having to buy another while I wait for a replacement from Evga, because I can't be without PC for weeks. If it's the mobo I'm going to buy a new mobo and CPU. The rest won't need replacing. Either way it's a pain in the arse. I flashed the BIOS earlier too with a newer version. I've also ordered one of those PSU testers from Amazon. Good ratings so we'll see tomorrow when it arrives.

Well I live in the UK. I can get and afford a replacement PSU no problem, it's just the inconvenience of it all, especially when you pay for top end stuff and it fails. I've rebooted several times today and it seems to work fine, ran the MSI Dragon Center tools and the 12v,5v and 3.3v are reading fine. Could be the mobo? It's MSI, never had any problems with them before and are renowned for their reliability. That's why I buy their GPU's and MOBOS. The PSU tester is arriving from Amazon tomorrow. We'll see then.

Well since I unplugged and replugged the cables on PSU, now more power or startup problems. PC hung and froze again on Sunday evening. Flashed BIOS back to and older version from 2019 where I never had any freezing problems and It's been fine since. I even left if on streaming all night when I was sleeping and it's been on all day today and working fine. I don't know what to say really

I've run memory tests, downgraded BIOS, updated drivers. I ran it again overnight last night was fine for many hours then hung again. Not only that, but I looked at event viewer toady and there are 29 critical errors going back to mid-Nov 2020 all relating to
 
'The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.'
 
Event ID 41 which on further investigation is 'The Kernel-Power critical error is usually power-related, and indicates something wrong with the computer's PSU (Power Supply Unit). Although this is due to me hard rebooting the PC.
 
I'm going to have to RMA this PSU.

tulit
So is there a problem with the thermal sensor on the PSU? 

All I can say is the Operating Temperature for an EVGA SuperNOVA 750 G+ is 0° to 50° C in the Specs..So if you think it's temperature related you can Contact EVGA about an RMA.
 
But like I said before, you could buy an el cheapo (or otherwise) PSU that's just good enough to get by on and put in it and see how it works..Never hurts to have a spare PSU around anyway..Just don't use the same cables, use the cables that come with the PSU.