Arc from 1Be plate cap lead to high voltage cage

[ShartZenith]

Working on a Zenith round tube TV. Fired it up for the second time. After it ran for about 30 minutes we heard a noise. I saw a blue arc inside the high voltage cage. After opening the cage it appears the arc was jumping from the plate cap lead of the 1B3 tube to the inside of the cage where the flyback live. Why is this happening and what can we do to eliminate this problem? Thank you.