[old_tv_nut]

"as the brightness value gets higher, the lines seem to disappear because (I assume) the voltage level on the raster overtakes the resolution of the lines (causes it to bloom over the bars)"

Now I wonder if we're looking at the same thing. I have been assuming you have been talking about the fairly narrow dark and bright vertical bars that extend at least a fifth of a way into the picture from the left. These are far too wide for spot blooming to average out unless you are seeing the picture go terribly out of focus at high brightness. They could, however, be a power bus or ground transient coming from the power supply that reduces in amplitude as the power supply is loaded more heavily.