@Lothar baier
Yes, that's a hunch of mine, too. The waveforms I'm seeing look very much like those one would see if there was a flakey regulator or smoothing cap supplying that part of the unit. It would be simple enough to check but for the fact that access in this area is so difficult there's no way to get a probe in there with the unit powered-up and I don't have any expander boards worthy of the name. I'll find a way, though....
Jinxie