No, I do not believe it's a power supply issue, as it has been working properly until now.
Based on the theory of operation.
Part of the power supply is always on (24/7) and supplies some power to the Main board processor and the Infrared remote receiver module (e.g. waiting for the for a 'on' command from the remote control or from the power switch).
When the power on command is received, the processor on the main board, will send a turn on signal to the power supply board, which then turns on the entire power supply to operate the set.
It seems that the turn on signal from the main board is intermittent, likely due to a faulty cable connector connection (or good ground contact where the boards fasten to the chassis).
Therefore, make sure that all the cable connections, on the Main, TCON and power boards are properly seated and make good contact.
When plugging/unplugging the cables, always handle with care, only via the connector body after making sure any release/locking mechanism is free (pulling on the wires can damage the cable).
Sometimes they get dirty (oxidized) and you could can clean the contacts with a Q tip and Isopropyl alcohol, to carefully wipe the exposed pins.