SQ80 display issue - works after 10 minutes?!

I have an SQ80 that displays junk (all segments on) when turned on. The Vbb rail (normally 50V) drops down to 18V. With the display board disconnected, the Vbb rail is totally fine. I disconnected each of the display drivers - both column chips and the 4 row chips to see which one might be dragging the Vbb rail down. I also disconnected each capacitor on the Vbb rail and measured it. All of the row chips were fine, and each column chip when connected will drag the rail down from 50V to 24V… very strange.

I decided to replace the column driver chips with some replacements from a pinball supply store but got the exact same issue.

What’s even stranger is that I’ve left the synth on a few times for longer than 10 minutes (display just showing “8888888888”'s) and turn it off and turn it back on, then the synth works fine!! (and the Vbb rail shows 53V)

Does anyone have any clue what might be happening here??