I just got my 74 type roadster running after twenty years. The old belts on the alternator screeched. Curiously every time I tightened them they quieted and then stretched. Almost two inches. So I replaced them. I tighten the new ones and they are quiet then they start screeching again. They also are stretching. I have never experienced this before. First how can new belts continue to stretch. Second, is it possible the alternator is giving resistance at higher speeds causing the belts to slip. The alternator seems to be working and I did not notice any resistance. I'm stumped.