Ok I appreciate you no doubt will have spent hours trying various things and different configurations along with 2 different mobo.
It is frustrating sometimes when you can't identify the specific fault but it is narrowed down now to either the PSU, RAM or CPU.
Try to google or search on this forum for your CPU debug codes specific to your system.
I can offer my help from personal experience of building PC s over the years and having faults that I have sorted usually with the help of others from forums like this one.
You have tried 2 mobo so it can't be the CPU header not providing power.
If you have a spare cooler then fit and try that with the CPU to see if it boots.
If not then use a working case fan, preferably the one fitted inside your PC.
Plug the CPU cooler connector into the case fan header.
Plug the case fan into the CPU header.
This way the mobo will still detect fans and cool the CPU but if your CPU cooler is faulty it won't stop the PC booting as the mobo will detect a working fan connected to the CPU header which is actually your case fan.
If it boots this way then the CPU cooler is faulty.
Some mobo will refuse to boot if a CPU cooler is not detected or it is defective.
It might spin the fans still but it might not detect it if it is faulty as there are 4 wires.
A PSU failing mid-game usually causes a system reset or just shuts the PC down.
A system crash is different to this though so it depends how your system actually crashed before during certain games.
A CPU failing or failed already is usually definite i.e. works or doesn't but not intermittently.
RAM failing can cause crashes, BSOD, freezing amongst other things and tends to be intermittent & varies according to use e.g. gaming can cause it to crash or benchmarking software that pushes the PC's hardware.
I would try a replacement PSU as this is the cheapest option and often a common fault.
I know you said it's a 750 watt so plenty of potential power but this is only if it's working ok.
If you get a PSU from Ebuyer you can return it as unwanted under their RMA returns system even if you have opened and used it.
I have bought and returned a lot of components from them.
Not sure about a CPU from them under the same system as it would have been covered with thermal paste.
If the replacement PSU doesn't work then it's either CPU or RAM that's the issue.
Both sticks of RAM wouldn't likely fail together, maybe 1 on rare occasions.
If you have a CEX shop near you then take your RAM in and pretend you want to sell it.
They test it, like all PC components, rigidly before resale so will tell you if it works.
Once they have tested it, usually 20 minutes, just say you changed your mind as the price they offered is too low.
They don't actually pay until testing is complete so it's not a refund.
These might seem a bit like unorthodox approaches but as it's a case of eliminating possibilites as it avoids buying new parts which might not even be faulty.
A faulty or failing PSU can provide low power and illuminate the mobo and power USB ports but fail to boot as it is a huge power demand.
A dead CPU tends to give the symptoms you are experiencing.
The fact that it booted before the mobo change makes it harder to diagnose so I hope this helps.
Andy