Ouch, a 10 year old OCZ. Wow. It still runs.
Umm ok, try returning bios back to factory optimized default and reset cmos. You upgraded bios, that was perfect, but at that time you had the i3 for a cpu. Bios is considerably larger than the settings that you can see, there's a whole bunch that isn't visible and was setup for the i3 when you swapped. With the hardware change, many of those invisible and visible settings get revamped to seat the new hardware, but sometimes some of them don't, so resetting bios to factory default can fix many/most/all of those. In the event one or two are still resistant to change, resetting cmos forces bios to do a full and complete hardware search/test/settings reset that will not happen with a windows shutdown. Between both, you basically tell the pc that you are the boss, and it's going to play nice and accept the 9400kf fully whether it likes it or not.
There's 2 basic kinds of psus, the old and the new. The old are group regulated, the +5v and 12v rail are grouped and regulated together, the 3.3v rail is seperate. It's an old design, has issues with uber low power modes and can't put out anything close to rated wattage on 12v, meaning a 500w group regulated psu on 12v rail might see 450w-460w. With almost everything powered by 12v in modern pc's, it's a kick in the pants to get shortchanged so many watts.
The new are dc-dc, and single regulated, all the rails are seperate, so there's little to nothing of 1 affecting the others. This allows for the really low wattages required by Haswells and newer Intel sleep states, as well as what the Ryzens require. It also means that a 500w psu stands a really good chance of seeing 500w on the 12v rail, or at least 480-490w. Actually getting what you paid for.
If you equate group regulated as being analog and dc-dc as being digital, you'd not be far off.
Not to say group regulated psus are bad, there's still some very good examples out there, but with modern demands of cpus and gpus for nice clean power, those older psus are getting left behind.
My motor in my car puts out over 450HP and gets 22-23mpg. Back in the late 60's when it was first introduced, it barely hit over 300HP, and got 6-8mpg. Times change, carberator vrs fuel injection, advances in combustion chamber design etc. Great motor back then, way better now.
And you have a mediocre psu, with all its group regulated shortcomings, capacitor degradation, wear and tear, serious lack of good protective circuitry, on some very cheap and questionable components. The word time-bomb applies.
Strongly.
VRM's are part of the voltage regulatory circuitry that are under those heatsinks surrounding the socket. The cpu demands 1 voltage for one thing, another for something else, and several others for other things from the motherboard. All those voltages, like vcore for example, are sent from the motherboard through the voltage regulatory circuits to the cpu. A nice, clean voltage is easy to work with, doesn't require much adjustment by adding/subtracting current/amps etc. A dirty voltage has high amounts of ripple, lots of ups and downs, so requires a lot of work to smooth out, and can still bounce all over, putting a lot of stress on the VRM's and cpu, and is a major cause of instability leading to bsod. For absolutely no apparent reason. Mid game, bsod nt-kernal error.
To put it bluntly, when comparing a new, good quality psu to your current psu, instead of washing your car with a nice clean rag, you are using 80-grit sandpaper instead.
At 10 years old, you are well past advise to replace sooner or later, and we'll into you should have replaced it yesterday, last week would have been better.