The Collaborative International Dictionary of English v.0.48:

Vacillate \Vac"il*late\ (v[a^]s"[i^]l*l[=a]t), v. i. [imp. & p.
   p. Vacillated (v[a^]s"[i^]l*l[=a]`t[e^]d); p. pr. & vb. n.
   Vacillating.] [L. vacillare, vacillatum; cf. Skr. va[~n]c.]
   [1913 Webster]
   1. To move one way and the other; to reel or stagger; to
      waver.
      [1913 Webster]

            [A spheroid] is always liable to shift and
            vacillatefrom one axis to another.    --Paley.
      [1913 Webster]

   2. To fluctuate in mind or opinion; to be unsteady or
      inconstant; to waver.
      [1913 Webster]

   Syn: See Fluctuate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

vacillate
    v 1: be undecided about something; waver between conflicting
         positions or courses of action; "He oscillates between
         accepting the new position and retirement" [syn: hover,
         vibrate, vacillate, oscillate]
    2: move or sway in a rising and falling or wavelike pattern;
       "the line on the monitor vacillated" [syn: fluctuate,
       vacillate, waver]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "vacillate":
   alternate, back and fill, bob, bobble, careen, change, changeable,
   chop and change, coggle, dally, dallying, dangle, demurring,
   dickey, dillydallying, dither, double-minded, doubtful, doubting,
   ebb and flow, eccentric, equivocate, erratic, falter, faltering,
   fickle, fiddle-faddle, flounder, fluctuant, fluctuate, fluctuating,
   flutter, go through phases, halt, halting, hang in doubt,
   hem and haw, hesitant, hesitate, hesitating, inconstant,
   indecisive, insecure, irresolute, librate, lurch, mercurial,
   nutate, oscillate, oscillating, pendulate, pendulous, pitch, reel,
   resonate, ring the changes, rock, roll, rootless, seesaw, shake,
   shaky, shift, shifting, shilly-shally, shilly-shallying, shuffle,
   stagger, stop to consider, swag, sway, swing, teeter,
   teeter-totter, tentative, tergiversate, think twice, timid, toss,
   totter, turn, uncertain, unresolved, unsettled, unstable, unsteady,
   unsure, vacillating, vacillatory, vary, vibrate, volatile, wag,
   waggle, wave, waver, wavering, wax and wane, weak, weak-kneed,
   whiffle, wigwag, wobble, wobbly