When you measure voltages you need to use the cold ground.
The tuner shield is a sure bet. Almost all the voltages come off a
flyback winding ( s ). HV looks normal.
Since you got reaction hitting it look at the jack pak connectors for cold joints.
Also check the whole chassis for heat areas for cold joints. Then
re seat all connectors. You can also tap around now & see if it comes & goes.
After that you will need to trace the signal through the video. I would use
composite for that.
Knowing where the OSD comes in will tell you anything after that is OK.
See if you can figure that out.
BTW a LONG shot but run the model & chassis numbers & see if anyone posted a TRAINING manual.
73 Zeno
LFOD !