The AMD cards have a feature that throttles power use as per demand. So I do not think it is not enough power at boot (very little or no gpu demand). I would remove the card and check the slot to make sure it is clean and free of and dust or dirt. Then re-seat the card making sure it is in the correct slot (usually the one farthest from the edge of the board). If that does not do it and the system worked with the previous card, you may have a bad card (RMA it).