"MidiOx is essential
here to test everything, and to display faulty sliders or datapots that
throw out spurious data randomly (the equivalent of a scratchy pot on a
guitar amp). "

Hmmm. Got a Yamaha that is stuck in sustain as well as port-something that I can never remeber the name. Where it slides from one note to another. I checked all the way form the jacks to the micro and the pulses seem to be right.

I wonder if a valid test would be to disable any and all MIDI to eliminat that as the source of the problem. I was kinds thinking the micro itself but am not sure enough to even think about ordereing one, if one can even be had.