Root, had/having some similar problems with my 52 w/700R4 tranny. The speedometer is good, checks good with drill ect. Took a ride with the speedometer cable hanging under the dash. It turns, then stops, them does it again, jumping, stopping.
I have a new cable!
Don't know about your SM420 but when this happens, in my experience, it is usually the plastic gear the cable gges into.
.....Once, on my 37 it was a simple fact the NEW speedo cable was to SMALL on the square end. Took to a friend how does this kind of work and he made me one out of the CORRECT size and have never had a problem in the past 10 years. (cheap china stuff)
May not help you but just saying.......
