89 F-150 6cyl 2WD cruise control

I believe that the speedometer cable could be binding up. Disconnect it from the speeometer and pull it out of the housing. You may have to disconnect it from the cruise contol module under the hood. Check the cable to see if it is binding up or has rust on it. Use light sandpaper and clean any high spots, put lube on the cable and reinstal. Also do the same from the cruise control to the transmission. When the cable starts binding up it makes the speedometer jump and become erratic. Check the vacuum lines to make sure they are not split or leaking.