My best guess is that there is a mechanical problem eg a switch or a plug/socket connection rather than a failed component. Switches may improve with use, plugs and sockets may benefit from removing and reseating or light cleaning with IPA or Deoxit. There is a thin coaxial cable from J2208 on the Zaxis board that goes to J4408 on the A6 Logic Board via the main interface board that the plugins mate with. If you can identify any of the plug/sockets on this signal and reach them easily, just pull the plug out and (carefully, it is easy to bend the centre connector) reinsert it.
If at any time the trace goes back to its original intensity then return R2135 to its original position (the exact setting is not critical, you should not be able to see the trace at minimum brightness setting on the front panel controls).
Can you now see the retrace (flyback) line where the spot returns from the right side of the CRT to the left side?
Do the A and B front panel brightness controls both work?
The advice I have seen here is that the safest way to use a storage mode CRT for everyday purposes is to use variable persistence mode with the persistence set to minimum. If you use normal non-store mode don't let the brightness increase to the point where the trace increases in width but not in brightness.