I eventually got fed up and took a gamble replacing the intermittent EL-backlit LCD screen with a modern 20x2 from Crystalfontz:
I'd seen reports that it's finicky about aftermarket displays but this one fired up just fine, and the problem hasn't recurred. The old display panel seemed very finicky about glitches on the +5 rail and I tried a lot of things but couldn't get it to work consistently, maybe the new EL backlight under it was interfering with it somehow, who knows...
But in the process of mounting the new display I managed to crack part of the cheap brittle 40 y/o phenolic board and had to jumper some power lines to the meter LEDs, ugh...this thing had a lot of problems to begin with and like it was definitely built to a budget, stinky single-sided phenolic with jumpers and air wires everywhere, it's from 1987 but built like a piece of cheap 1970s home stereo equipment, with wobbly input jacks and controls mechanically secured only by the PCB.
Kind of a PITA to work on, but it is almost fully digital with only a couple LM13700 and companders on VCA duty so there's not too much in the SM to adjust once it's running..
The Sony digital reverb unit I have from a few years later (1991) is built like a piece of aerospace hardware by comparison. I've only seen a couple of these for sale domestically in the US over the years, seems like they didn't make too many and they were all screwed-up to begin with like this one was so I wouldn't be surprised if it was one of the few still in operating condition. Sounds nice, but I don't really want to deal with one again like this haha
It does look cool all lit up. Uses an NEC variant of the Z80 at 12 MHz for brains.