IIUC you're playing a soft synth by a keyboard, connected via a hardware
MIDI interface, when you get audible "drop outs"?
If so, does the MIDI interface and or the audio device share the
interrupt with something else? If so, did you unbound or remount
hardware to get rid of shared IRQs?
Did you increase Frames/Period?
What's the output of /path/to/rtirq status?