You cannot measure with a multimeter if this is really stable all the time. There may be short voltage drops. You are near the limit af the A4988, as it needs at least 8V Vmot. More is better - it should be at least 12V.
Furthermore I don't see see the needed capacitor of at least 47µF ( more is better) close to the Vmot and Gnd pin of the A4988.