Horizontal issue with a mid 60s Silvertone (video attached)

[zeno]

Sure sounds like HV arcing. Use a paper towel tube
to find where its coming from first. The arcing can cause
the sync to mess up. Check at anode, FBT, yoke &
especially be sure the dag is well grounded. A set will
run with a floating dag & you will get the light ticking as
the HV makes little arcs to the loose ground. Can break up
the pix or cause stripes of dots in pix.

73 Zeno