How old is the PSU? If you bought it back when the 780 ti came out then it has probably degraded over the years and can no longer put out the same wattage anymore.
looking at your PSU it only has 2 6pin and 2 6+2 pins, the kingpin needs 3 8pin plugs to operate. I would look into a new PSU
SeaSonic FOCUS Plus Gold 750w
EVGA SuperNOVA G3 750w
Corsair RM 750w
PCPartPicker Part List
Power Supply: Corsair RM (2019) 850 W 80+ Gold Certified Fully Modular ATX Power Supply ($118.98 @ Newegg)
Total: $118.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-11-20 13:21 EST-0500