Is your spot killer on , and if it is ,turn up your brightness and look for a white dot on the screen ,if there is your high voltage cage may be bad, may need to do a cap kit
1) check all deflection board fuses
2) I would play with the brightness and contrast pots while on
3) unplug monitor, check pins for damage and corrosion, plug it back and retest
4) pull the deflection board and re-flow all the header pins and check for shorted C100 or C101
5) recap deflection and HV board (including C100 and C101) and replace pots (why not while it is out!) and replace all four of the chassis transistors.
Sorry I have been a little under the weather the last few days and have not had a chanct to work on the game. I will try some of your ideas and let you know.