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.