Or search: flutter, commotion, contagion, happening, infection, kerfuffle, omissible, occurrent, disruption, occurrence, peripheral, disturbance, parenthetic, transmission, parenthetical
2 Syllables: SU
interim
3 Syllables: SUU
immigrant, implement, impotence, impotent, impudence, impudent, incidence, incidents, increment, indolence, indolent, inference, insolence, insolent, instrument, insulin, lineaments, simpleton, stimulant
4 Syllables: USUU
coincident, Dominican

Found 22 words across 3 groupings. Skipped 4 excess words of a similar stem.