When I install a blade in my Dewalt, I put it in the bottom clamp first. Then I pull down on the top arm so that all the slack is out of it. Tighten the blade clamp. Then, tension it. I usually have mine set at about 4 or so depending on the size of the blade. It sounds like you're not getting the slack out of the arms/mechanism prior to tightening the top clamp. The same goes for when I loosen the top clamp to put the blade through the next entry hole. Then do the process all over again. If this doesn't cure your problem, then it's time to take the saw apart.
R