[old_tv_nut]

The schematic says the grid of the 6AC7(pin 4) is -2 volts.
I don't think it would hurt to temporarily ground it, so I'd try the following:
1) put a scope or meter on pin 4 and see if it jumps around when the bad waveform occurs
2) if it is jumping with things normally connected, it could be caused by anything from the discriminator to the oscillator because they are all in the loop. I suspect you will see it jumping around.
3) ground pin 4 so the control voltage is constant = zero. Hopefully this only throws the frequency off some.
If the oscillator is now stable, you have isolated the problem to the discriminator. If the oscillator is not stable, the problem is in the control tube or oscillator.

Another thought - with set off, pull the Horisontal Oscillator (6K6-GT) and measure the resistance from pin 5 (R196/C172), looking for intermittent while adjusting Horizontal hold.