The 780 looks to be 5 years old so I'd remove it for now and try just onboard graphics. If that works, get out the canned air and do a nice cleaning (what is canned air and how to use is on many youtubes.)

Try using DDU to remove all old GPU drivers than try again with Nvidia's GeForce Experience.

Also, while you work over the PC, be sure all fans work like new.