[SiriusHardware]

Can you confirm:

You're using MIDOX as your MIDI analyser / receiver

This problem (occasional transmission of the right note on the wrong channel, along with occasional failure to send a note-off message), is happening on ALL of these combinations:-

-Keyboard connected to low end computer with cheap USB MIDI interface

-Keyboard connected to high end computer with high end MIDI interface

-Keyboard connected to a multitimbral tone module, bypassing computers and MIDIOX altogether.

Failure to send note-off could be a purely mechanical problem if each key uses a single-pole double throw switch, with the common connected to one contact when the key is at rest and the other contact when the key is pressed.

The reason it might work this way is if it is velocity sensitive, so then the keyboard scanning regime measures the elapsed time interval between the ending of the 'key at rest' state and the beginning of the 'key pressed' state. The shorter the interval between the two states, the higher the velocity. In an arrangement like this it is quite possible that the contact for the 'at rest' position has become tarnished and does not always 'make' when the key is released and returns to the rest position.

Or, your transmitted MIDI data is so badly deformed that MIDI receivers are just having trouble understanding it. You really are going to have to try to scope the data, if possible.

Here's another idea. People sometimes use 'Audacity' (the audio toolbox program) to capture DATA rather than audio, using it, in effect, like a one-shot storage scope. I've seen an article where someone captured the output from a 433Mhz remote-control socket receiver using Audacity in order to understand the data format. Maybe you can try something like that? You would probably need to attentuate the data level and put the audio input on your PC into line-input mode.