Does it modulate the size in TEST MODE too?
Is this a working board that just recently started doing this?
Have you looked at the voltages etc? Reseated the monitor harness etc? Cleaned the X/Y pots including SIZE?
My guess is monitor. Looks like you're getting deflection collapse based on the amount of stuff drawn on the screen -- less stuff, more collapse. Is your spotkiller flashing on?
Nice lookin symptom, that tube's puckering harder than my asshole when I cum.