Dave:
I would the small electrolytic capacitors that are part of the
driver circuit for these transistors. They are usually located
nect to the driver transformer. Use an ESR meter, or just replace
them if in doubt.
Many better monitors use an FET to regulate the supply voltage
going to the flyback and sweep transformer. Sometimes the FET
will short or the electrolytic capacitor will weaken causing the
FET to fail to regulate properly.
Of course, the flyback (LOP transformer) might be going bad,
which means it is time to look for another monitor.
Hope this helps...
John