I realize this might not be terribly helpful, but I have encountered a pair of similarities with my plain 494 that I have been waiting to worsen so that I might better trace the problem.
When first turned on, the screen characters, but not the other traces, on my screen exhibit a vertical shake of perhaps 1/16-inch. This gradually subsides in less than a minute. But then when reducing the Frequency Span, in switching from 500 kHz to 200 kHz, the screen displays "Phaselock Failure -- 1st LO."
Now by turning the power off and back on again, everything works OK and the analyzer will phaselock properly down to a resolution bandwidth of 30 Hz.
The voltage test points, near the left-front, show no evidence of jitter. I have not changed the power resistors on the deflection board, as recommended by John Miles, but now have replacements on hand. It is possible these resistors in the vertical deflection circuit are arcing internally, but eventually heat and heal.
Interestingly, if the Peak/Average control is turned all the way CW so that the horizontal line is off-screen at the top, the screen characters will jitter vertically. I assume this is an intentional warning feature that is digitally generated, and unrelated to the fault.