maybe a weak HOT. did you check the shunt tube current
remember how to do that.
when you have problems with HV, 1st thing to do is sub in new tubes (damper/HOT/shunt/HV rectifier).
clearly you HV is dropping in response to beam current, so either there is too much beam current or not enough HV in reserve.
If the pic is dim 1st thing to check besides the brightness and CRT is the video out tube, as the CRT cathodes are directly coupled, a problem there will cause bias issues in the CRT.
did you check the HOT drive voltage? if its more neg than it should be then you may have the blanker tube grid resistor trace issue.