that the issue of instruments going missing at random still exists. This is most noticeable with horns.
This is polyphony related. They're not going missing "at random", we know they're going missing due to polyphony droputs. At times, the SC-55 has half of the polyphony required, as estimated earlier.
Depending on how partial reserve is set up, notes will drop out differently, as mentioned by Falcosoft.
The French Horn goes missing most frequently as it is a 2-voice instrument (SC-55 patches use 1 or 2 voices PER NOTE). The SC-55 has 24-28 *voice* polyphony, not note polyphony, thus this is reduced to 12-14 *notes* when using 2 voice patches, for an overall note polyphony of 12-14/24-28, depending on the hardware and the mix of 1/2 voice patches used.
The overall finding of this thread can be summarised thusly: "Playing MIDI files on synths with insufficient polyphony for them leads to somewhat random dropouts." Although this can seem weird, as in the EverQuest case, devices like the SC-55/variants are made to handle a few notes over and still sound convincing, not 30 notes over. 😀 So you end up with different results depending what notes the synth cuts and what goes through. As Falcosoft said, MIDI timing can cause variations in this, as does device unique firmware and settings. This used to drive me mad with the MT-32 before I knew what caused it!
What would be interesting is to hear some known Sierra/game tracks with more usual polyphony issues, that were composed FOR the SC-55 (not for a soundfont with unlimited polyphony). I think one of the Castle themes in QFG4 (the one with Space Voice) and the Bookshop track in GK1 are the two Sierra ones I know of. Falcosoft probably knows of good ones like the one he posted.
And then, see if the variation still appears between devices. I believe a mkII solves all the known Sierra examples, for instance.
GK1 track attached here, you can see (hear?) that like the EQ track, it has a lot of sustained notes.
The issues are present after about 30 seconds, in the "main" part of the track.