fickle

adj.

[1]

volatile , flighty , capricious , frivolous , unpredictable , moody , giddy , fanciful , whimsical , fitful , mercurial , volatile , unstable , changeable , mutable , inconstant , changeful , unsteady , unsteadfast , indecisive , undecided , vacillating/rarely vacillant , unsure , uncertain , irresolute , wavering , erratic , unreliable , undependable , irresponsible , untrustworthy , faithless , unfaithful , disloyal , (Colloq) wishy-washy

[2]

erratic , mercurial , quicksilver