The sounds I tried all were piano type sounds, so perhaps therefore the problem didn't show or not enough to hear.

I found the following on the net about XP-80 sequencer fixes. Does anyone have an XP-80 with ROM version > 1.04 and still have these problems ? I still like the XP so I hope this gets fixed.

XP-80: ROM 1.10, CPU 1.04 are the latest versions reported.

CPU 1.02 fixes a problem with the PR-C:098 patch. Sometimes when played 'popping' and 'clicking' can be heard.

ROM 1.10 fixes a bug when assigning GM patches 041-056 to Performance
PR-A:01 Part 1 no sound is heard. When loading and converting a SMF format 1, track 15 may be merged into track 16 and track 15 will be empty. In track info screen, track 16 _may_ show the information for a previously loaded song. When using soft thru and there are multiple note on messages, the XP _may_ only transmit one note off message. While recording the timing may have occasionally drifted.

ROM 1.08 fixes a bug when recording with the Quantitize function ON,
very rarely the XP may miss recording.

ROM 1.07 fixes a slave mistep bug, an external sync to clock bug and
a chain play bug.

ROM 1.05 correctes a timing problem with the sequencer syncing to patches (drum loops) from the Dance exp. board.
ROM 1.04 apparently fixed some sequencer timing problems when large amounts of MIDI data was being received.

To find out your current ROM/CPU version:
Press [SEQUENCER] button to enter Sequencer mode. Then press [SHIFT]+[ENTER]+[EXIT] simultaneously.