From
https://www.gryphel.com/c/mail/v9.html#m176:
I can think of two reasons for such jitter. First, OS X is not a real time operating system. The operating system will give Mini vMac time to run when it feels like it. To get uninterrupted output for the emulation of built in sound, there is a buffer that the operating system can get samples from, even if Mini vMac doesn’t get time for a while. (This buffering results in lag, which can be a problem such as for games, so Mini vMac dynamically tries to figure out just how much buffering is needed.) But this is probably not the main issue you are seeing.
Second, Mini vMac emulates an entire tick, about a sixtieth of a second (more accurately 60.15), all at once. If the operating system feels like it and runs Mini vMac exactly on schedule, this will run every sixtieth (about 0.0167 seconds). Your emulated Midi clock events are supposed to happen somewhat less frequently. So in most ticks one event is emulated, but occasionally there are no events in a tick. Which is exactly what you are observing.
The only way to avoid this would be to have the operating system run Mini vMac much more frequently, and each time emulate small pieces of a tick. This would be much less efficient, and it is doubtful whether many operating systems would even allow this.
If possible, a better option is to buffer output, like the emulation of built sound does.