The phrase "in and out" can be used to describe quick movements or actions, and there are a variety of synonyms that could be used in its place. Some possible options include "fast," "swift," "rapid," "speedy," "brief," "fleeting," "momentary," "transient," "intermittent," "occasional," "sporadic," "irregular," "unsteady," "wavering," or "uncertain." Each of these words carries a slightly different connotation, so the choice of synonym would depend on the specific context and intended meaning. However, all of these words can be used to convey a sense of movement or action that is both quick and brief.