What exactly happensIJ Do you lose your auger flightingIJ BearingsIJ Sheet metalIJ The common result of running the machine too fast is the sheet metal on the shaker shoe starts to fall apart.
When you say crank arm, it makes me think of the shaft that rocks the sieves. How does it turn by hand with the belt offIJ Any binding, looseness, etcIJ What is your rear beater rpmIJ This is the critical rpm for the machine, and I think the later 750's went from 710 to 740 rpm (I'm not sure about this, I'll have to check.)
The early ones were 710,720 but they changed to 740 fairly quickly,you can tell by the 740 tag on the rear beater pulley on the right hand side. They also changed that shaft to the same thickness as the 760's,860's as well because a few did break.Where the two bearings fit into the crank arm for the sieve box there can be wear and this causes the sieve box to jerk. We did build up one but it was easier to buy a new one.We changed a 750 to the bigger shaft(you need new bearing holders as well as a new crank if memory serves me right) that caused us some problems and after that it was great.Never had any problems with the later models.john