My '79 450SL has developed a shifting problem. When you start off, everything is fine until it is supposed to make it's first upshift. When the shift occurs (rather hard), the Tach will go down to 0 and then back up and then down to 0 again, and then the transmission will downshift to 1st gear again. It keeps trying to cycle like this and you can not get enough speed to shift to 3rd. When I try to manually shift from 1st to 2nd, the same thing happens with the RPMs and the transmission will basically just disengage until I pull it back into first.

Any ideas?