(adv.) Back and forth, like the movement of a shuttle.

alternate alternately back-and-forth backward and forward backwards and forwards capriciously changeably desultorily erratically hitch and hike in and out inconstantly mutatis mutandis off and on on and off reciprocal reciprocally reciprocative ride and tie round and round seesaw shuttlewise sine wave to and fro to-and-fro uncertainly unsteadfastly unsteadily up and down up-and-down variably vice versa waveringly