90% of the time, the super high resistive material in the G2/Focus block breaks down, and it zaps the electrodes in the back of the CRT. 

The only fix is to change the flyback, OR.... find an external G2/Focus block that goes in-line the HV lead from an old TV set. I think some of the early 70s trinitrons used them, and they MAY still be available. 

In that case, patch that inline, and cut the wires off the flyback. Done.