Hi,
i have a similar experience with HD channels and my aerial is good!, i was told by an installation engineer to hold on till the new year when the transmitters have stopped faffing about with HD testing and see if things improve, i have a signal strength of 30 but the quality is 100 but only on HD, on standard freeview they are both on 100, i may even consider changing my aerial again at a last resort, but it may improve yours if its never been done.