That covering holes was just for testing if its gpu related. It might be also one capacitor gone bad in motherboard, but troubleshooting this would require multimeter at minumum.
Just for the heck of it download memtest for usb / dvd and run couple of passes, if it wont give any errors we can say its not atleast memory related.
You could try doing mild reflow on the APU, nothing near melting solder but just to see if its failing connection inside chip which can be fixed for a short period without even going 220c temperature.
https
/www.youtube.com/watch?v=gvvQ10hlgb0 Dont do this long and high temperature, you just need 100-150c to test this, if it works for next 50-100 boots then issue is surely this.
Complete fix would be replacing chip if this is the problem.