chine walk

this afternoon, i rode in a buddies '75 20ft master angler with a closed transom and non flotation single armstrong bracket w/200hpdi. the boat ran great up to around 53mph, but when it trims out it really starts to dance- around 57mph it was uncontrollable and had to back off. no combination of tabs/motor trim helped. im in the process of building a '70 20ft SF that has already been enclosed with a hermco flotation bracket, and was curious about this chine walk. do all 20s have this problem over say 55mph or could it be a poor design with the enclosure and bracket? any input would be greatly appreciated!