3 months ago i replaced my speedo cable on my 89 iroc z28 because it broke at the trans end. the auto parts had only a one piece unit and not the two piece type that i had. i don't have the ecm or cruise control because i converted to carb ,have 700r4 auto trans. it worked and now it fluxes up and down. i read about vss in the gauge cluster plus some gear in the trans. what could cause this problem. i already checked the cable and it is connected at both ends, not broken.

 

Could be the cable is binding. Did you lube it up with a graphite bases speedo cable lube? Regular oil or grease will get gummy which makes the cable stick and the needle jumpy.