These are always difficult and very time consuming... because the problem could be almost anywhere... Memory, video card, driver, chipset, power supply, hard drive, cpu fan, thermal paste too thick or too thin, corrupted installed, conflicting security software, automatic updates creating changes that conflict...
Since it used to work OK at one time, I would start with that basic unit... and assume something is wearing out or getting too dirty. Don't forget the simple things like checking for flags in the Device Manager, changing the thermal paste.
If it is in your budget, buy a thermal gun for about $45 to check various temperatures. Also consider a quality power supply tester... about $25.
Then begin replacing the obvious components... power supply, cpu fan.
Don't rule out PCI devices you don't even use. We have seen audio cards and modems go bad and create spurious responses. Of course the big problem area very often see is the cooling fan on the video card.
The more expert we are, the more we ignore keeping a log, but that can be very helpful as failures continue. Do not forget to audit the event viewer.
I know this isn't much help, as you really want a fix. But most of us know the problem will not go away quickly.
Let us know how it all turned out.
