Re: Computer playback
<snip>
* It results from accidental transposition (by a semitone) of that channel and can be temporarily corrected by re-transposition,
I usually side-step the problem by shifting from Channel 1 to another channel (or by changing to the Viewer for playback).
If the problem occurs early in the notation of a new song I change that staff to another channel.
<snip>
Have you tried pressing <F6> a couple of times instead? <F6> stops playback, BUT if the score is not playing then it performs a reset of all open MIDI device ports (according to the command reference).
I've also noticed that several presses when not playing a score will open a dialogue box asking if you want to sent a full reset for all notes on all channels.
Also, out of curiosity, what synth are you using and has it been the same one all along?