i have a 1994 ford 7.3 turbo diesel IDI non-powerstroke. the speedometer starts jumping around after driving 20 or 30 miles at 70 mph. 65 and below i don't have a problem. i have changed the speed sensor at rear end twice ( purchased a o'reilys). i took out the ECM and sent it in to be rebuilt. speedometer still messing up. the speedometer bounces around all over the place below 70 and if it ever goes to zero it messes up the transmission. causes it to shift very hard. have to shut down and disconnect both batteries then hook them back up and transmission works fine....E40D transmission.....need help

 

1994 F 350 7.3 turbo diesel IDI...does the speedometer get it's signal from the RABS sensor... if not then what does control the speedometer