I am wondering psu.
Reason for this is certain psu connectors and/or cables are strictly for graphics supply..
How old is the PC?
Could it need the dust blowing out including from inside the psu.
I would consider carefully disconnecting/connecting each psu to mobo connector 2 or 3 times, which should clean them.
Do some googling first on what each connector supplies.
If a graphics one is very tight, it could be it is being overloaded.
Good Luck - Keep us posted.