Capacitor going bad in the HV circuitry?
Now that I think about it I'm asking why I didn't check. I was blaming the RF section because it drives the display, maybe it's the display section.
I would start there. From my (admittedly limited but increasing) knowledge of troubleshooting HP gear, my progression is always mechanical stuff first (contacts, pots, etc) first and then suspect anything that has contact with mains and then higher voltage circuits before suspecting any low voltage circuits. The former two will always be under more stress than anything on the low side.