Hi John,
Look for a capacitor in the filment supply of the CRT. You'll probably
find one bad causing almost no filament voltage when the monitor is cold
and as it (the cap) heats up, the filament voltage slowly increases to 6.3
volts or close. Find the filament terminals on the crt socket on the neck
pcb then trace the apropriate wire back to the main PCB and you should
find the right capacitor pretty easily.