Chevrolet Repair: 1985 chevy s10 blazer speedo bounce, chevy s10 blazer, s10 blazer

Question
i am currently working on a 85 chevy s10 blazer with the 2.8 automatic in it and I'm having problems with the speedo bouncing very sporadically do you know what the most likely cause is?

Answer
This is usually the speedo cable or the speedo gear inside the tailshaft housing of the trans.  The easiest thing to do is to remove the speed sensor from the outside of the tailshaft then remove the driveshaft.  Turn the trans output shaft to see if any teeth are missing on the speedo gear.  If so replace the gear.  If it looks okay your problem is in the cable or cluster
Jimmy