Or search: bust, gnaw, rust, wilt, decay, droop, erode, tumble, corrode, crumple, collapse, dilapidate, disintegrate
2 Syllables: SU
bumping, bungling, bunting, coming, Cummings, cunning, drumming, dumping, fronting, fumbling, funding, grumbling, grunting, humbling, humming, hunting, jumping, mumbling, plumbing, plunging, pumping, pumpkin, punching, rumbling, running, shunning, something, stumbling, stunning, summing, thumping
3 Syllables: SUU
blundering, comforting, functioning, lumbering, numbering, plundering, slumbering, thundering, tumbling, wondering
3 Syllables: USU
becoming, confronting, forthcoming, succumbing
4 Syllables: USUU
encompassing
4 Syllables: UUSU
unbecoming

Found 47 words across 5 groupings. Skipped 1 excess word of a similar stem.