It can be the PSU but I've also had to replace a motherboard over same failure. The PSU at 7 years did fine. Try another.
When I press the on button to start my computer the lights come on for about 5 seconds and then turn off. If I hold the reset button and the on button it starts up but it does not boot up. It just tells me what kind of motherboard I have.
When I turn off the power on my power supply and leave it off for about two minutes and then turn it back on again and then turn on my computer with the on button then everything works fine and it boots up normally.
Does this sound like my power supply is starting to fail?
I built the computer myself 7 years ago so the power supply is seven years old. Should I install a new power supply?