I recently experienced a known problem on a 4FG monitor. Vertical
deflection was unstable and sometime it did collapse while. Of course I
did look for any capacitor leakage in the vertical stage, as usually
happens with this monitor, and naturally I did find a couple of them
that released capacitor fluid on the PCB.
I changed all electrolytic capacitor on the vertical stage but vertical
problem was not fixed. I did also find cold welds on IC552 and Q552 but
neither this was the cause of vertical jitter.
Then I started to investigate the vertical stage circuits finding that
jitter was due to vertical sync noise on pin 4 of 555 timer on PWE
317-C module (pin 10 of the module). Following the signal route I
reached the pin 11 of PWE 317A module and suddenly I noticed that
vertical synch on that pin was non noise affected! Checking the wiring
track between pin 10 of PWE 317-C and pin 11 of PWE 317-A I measured
about 1000 Ohm!!! This was due to a bad connecting hole on the PCB (the
one near pin 11 of PWE 317-A); remaking the wiring track by an external
wire the problem disappear and now the monitor is perfectly working.

Hope this can be usefull.