sounds like you could just be tightening it in the wrong order.
Try loosening the bolts on the stem first, then tighten the allen bolt on the top cap, this should tension the headset, check the stem is straight and tighten up the bolts on the stem again.
sounds like you could just be tightening it in the wrong order.
Try loosening the bolts on the stem first, then tighten the allen bolt on the top cap, this should tension the headset, check the stem is straight and tighten up the bolts on the stem again.
^top cap