I had intermittent boot and power related issues with my Gorgar - solenoids would also sometimes loose power in the middle of game and I would have to turn off and back on to reset.
I made the wiring adjustments recommended in this old service bulletin and have had not issues since - https://www.ipdb.org/files/1062/Williams_1979_Gorgar_Service_Bulletin_SS20_no_date_special_adjustments_and_wiring_change.pdf.
Check the voltage from the power supply to the MPU - it connects at IJ2 on the MPU board.