You could check the capacitors on the mobo. If the mobo is the problem, and you're lucky, you might see one bulging, meaning mobo needs to be replaced. If is the mobo, and you're unlucky, you will not see anything, making diagnostics more difficult.
Assuming no changes have been made to the system, it's usually a component that recently failed. Assuming the fixes you already tried were with good components, sound more and more likely it could be the mobo.
one more thing to check: if the CPU overheats rapidly after PC turns on, it can cause the same thing (although usually it just turns the system off). Check the cooling for that CPU (radiator and fan) and give them a good cleaning. I doubt that is the case for you, but it can not hurt anything to do it and eliminate this possibility, however unlikely.