Run capacitor replacement

[Electronic M]

I suppose it is possible for it to be intermittently leaky and consume enough power to drag input voltage to the set down and reduce brightness, but that somehow feels like a big stretch.

If the brightness problem persists it may be good to swap the output transistors, or at least monitor HV and gun voltages, preferably with a repeatable test pattern on screen, before and after to find out which line is changing. I mention video output transistors because on a 70's Zenith hybrid I had problems with loosing color from one gun. It was in brief intervals, and seemed to be part random, part temp and vibration sensitive....Would change as it warmed up would correct with cabinet slaps, would become insensitive to cabinet slaps but be sensitive to tuner action, would be sensitive to sound, etc. Damn thing would never have the problem when I could meter it, till it was practically permanently stuck with a dead color (took 2-3 years of living with that intermittent to get there), once I could watch all gun voltages and track the effected line diagnosis was easy.