Question PC shutdowns 2-3 seconds after I press power button & after 3-7 shutdowns it boots ?

[restartc93]

PC shutdowns 2-3 seconds after I press power button.
Usually after 3-7 times it boots with "BIOS checksum error" and automatic recovery. On lucky day it boots from first time.

I replaced CMOS battery (small round in motherboard) but it didn't help.

How to fix this issue?

I have 11 year old pc (Athlon || x4 635, GA-MA785GT-UD3H) and this issue started out of thin air.

 

[rgd1101]

brand and model of the psu?