As you say, it might be a PSU issue - but it'll be easier to change graphics card first. Which is what I'll do on my return (after I've completed some more memory tests). I'll let you know how it goes.
Not sure if this will help you but it sure helped me!
I was experiencing the exact same issues, LOOOONG boot times (up to 35 minutes!) and random freezing with a hard reboot the only option. My PC specs are way above what is needed so it definitely wasn't that.
Every reboot my PC wanted to scan & repair my "H" drive which is where I store my games & backups. It is not listed as a boot drive in the BIOS so the only thing I could put it down to was the backups that were causing an issue. I deleted all traces of the backups and rebooted and lo and behold, my PC was performing as it should've. Boot times were back to 15 seconds on my SSD and freezing is a thing of the past. (Haven't had a freeze in 3 days yet)
FYI, these are backups created by Windows Backup & Restore.
And I've not had a freeze in over a week. So I think that's that, it's solved. :-)
So, it looks like the problem (in order of likelihood?) was either:
Unwell graphics card (and I guess upgrading to Windows 10 might have brought a minor/unobserved problem to the fore - a bit like changing a part on a classic car engine, which then has unforeseen ramifications elsewhere!)
or some weird Nvidia-ish driver issue (the new card is an AMD)
or too much heat building up in the case (unlikely, as I was monitoring it, but I suppose it's possible...)
Anyway, I will mark this as Solved. Thank you for your help!
I've lost far too much time on this to check whether the card is actually broken (I could install it in my daughter's PC, but she needs it for study, and I don't have any other desktops at the moment). However, I will be selling it on eBay - for Spares and Repair - to recoup a bit of the money spent on the new card. Perhaps the buyer will let me know if it works perfectly, and they've got a bargain (in which case, I'll try not to be too frustrated... ;-) ).
Of course, this being Windows, I've had one or two issues since (notably explorer.exe getting into a loop of crashing and restarting last week - that was a bit of a nightmare), and being an inveterate tinkerer, I subsequently put some slightly faster memory in as well... But that's a story for another day.
It seems to all be working at the moment, and finally I can get on with some work!
Hoping it is not something else, but only time will tell for sure, Power Supply, etc.... Saying you added RAM which we thought was another cause of the issues, hopefully bought the correct type of RAM for your motherboard. Could of listed the AMD card you had purchased for us to make sure your PSU meets it's requirements. Overheating might of been mentioned before, as so much to reread but can be also a cause this is something should be done on a monthly basis with a desktop, clean inside of dust and dirt as it can damage sensitive electronics and fans by working them harder and overheating them.
Need more help you can post it here still i will know.
Seems I have been seeing a lot of "Freeze" issues lately, on my system it was caused by one of my main hard drives going bad. It would drop off the SATA bus, and only my mouse would work, I could not click on anything. I finally had to disconnect the drive. You might want to check all of your drives even USB's to see if any of them are dropping out.
Anyway, as I say, not a single 'freeze' since replacing the Nvidia card.
Thank you.Need more help you can post it here still i will know.