I have more data! It's not the tube. At least I don't think so.

This morning on the way to work, my display seemed to get dimmer. I think it actually did, but it could have just been a change in ambient light. Regardless, I went into the dimmer settings and lowered it all the way down, which is something I'd do before I installed the cap to fix the non-linear brightness control thing. When it got down to 30%, which was barely visible, the other half came back on. When I brought it up to 40%, the other half blanked out again. Weird.

I did attach a cap to the IC on the back of the display board and this seemed to fix the brightness adjustment linearity problem. However, it was not a SMT cap, but a throughhole ceramic disc cap. Would that make any difference? The specs were the same as the SMT cap suggested inthe FAQ, and I did not make this change to the display board that previously had this same thing happen.