Update: The issue at hand seems to be caused by a thermal problem, however the 1976 service manual screenshot 104 seems to match with my earlier uploaded picture of the white line, which points to a faulty PSU. I'm inclined to believe that the PSU is normal, and that the regulators may be in the wrong spot, as the UA7812 regulator is hot to the touch after the TV loses signal. Prior to the signal cutoff, with the regulator cold, the system works just fine until the regulator heats up, and likely causes a drop in the 12 Volt line? (Again, I'm not sure if the regulators are even in the right spot.)
I would greatly appreciate if masschamber could upload a pic of the regulator setup, because if the person who last serviced the machine put the capacitors backward, then I am likely to assume the regulators were also put in incorrectly.