trans problem on 1992 Chevrolet 3500 Pickup

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
the speedometer has been dropping to zero and keeps the trany in first. sometimes at idle it will jump around. i changed both speed sensors and checked continuity uf the wires from computer to tran. dont know what is causing this. any help would be appreiciated.
(1) Answer
The wires for the speed sensor may be grounding out some where between the speed sensor and the computer. Try and follow the wire harness from the speed sensor to the computer and look to see if the wires are damaged anywhere along the way. If one of the speed sensor wires is damaged and shorting to ground this will cause the speedometer to go to 0 whenever the wire touches ground.