Hi James,
What electronics skill level do you have?
The issue is likely to be in the switching or power supply to the high voltage (-28v) to the VFD (FL display) as the entire display is fluctuating, but appears otherwise fully lit and functional. The heaters are fine.
I would start in the power supply for the VFD, including checking the FL (VFD) switching and the zener regulator as per the two schematic captures I have taken below. These are from the model 8020, but I am hoping the general parts placement is similar to yours.
Check the voltage on D24/C11 and ensure the cap and the zener are good.
View attachment 1187662
View attachment 1187670
On the power supply, check C163, the 47uF 63v cap-
in fact, I'd just replace it as it could cause this entire problem. Check Q23 or equivalent is being switched on or off and doesn't have a fluctuating base drive to it. Check the -30v is stable on not full of ripple. (use a DMM on ACV/mV to see that or a DSO/CRO)
Assuming the main boards are the same on this model (I'm hoping) this is where c163 lives:
View attachment 1187674