Driveability

I own a 1996 GMC Sierra SLE. The spedometer is bouncing when letting off the accelerator and the transmission goes into neutral. I was told to check the wiring from the VSS back to the PCM. Have done that. I was also told to replace the VSS. I have also done that. I have torn the dash apart and taken out the spedometer and checked the three wires to and from the spedometer head back to its source and everything is alright. I am now at a loss as to what to do next. Could someone please point me in the right direction.
Thanks
Kirk Terrell