I've been blessed!

[old_tv_nut]

Changing the HV regulator had no effect.

I have determined that the delay line output (which feeds the high side of the brightness pot) doesn't drift, but the negative voltage for the low end of the pot as well as the wiper voltage does.

Now to do a sanity check on how much the drift is at the wiper vs the low end. The wiper should be a voltage-divided version of the negative supply drift. If it's larger, there's some variable load on the wiper.

If the drift is coming from drifting of the the negative bias generated by the horizontal output, it's going to be a problem to get at because all those components are under the chassis