WordNet sense 1 (be unsure or weak):

falter

WordNet sense 2 (pause or hold back in uncertainty or unwillingness):

hesitate, waffle

WordNet sense 3 (move back and forth very rapidly):

flicker, flutter, quiver, flitter

WordNet sense 4 (move or sway in a rising and falling or wavelike pattern):

fluctuate, vacillate

WordNet sense 5 (sway to and fro):

weave

WordNet sense 6 (the act of pausing uncertainly):

faltering, hesitation

WordNet sense 7 (give off unsteady sounds, alternating in amplitude or frequency):

quaver