[old_tv_nut]

Unlikely that your power connection has anything to do with it, since the raster is not shrinking when the video and audio disturbance occurs.

Reseating the tubes is a good first try, but if/when that doesn't work, the next thing might be to tap/prod various internal places, bend wiring, etc. with an insulated stick to see if you can find the intermittent item. Edit: I think I would start with the cables connecting between the tuner and the main chassis.