1992 1500 4.3 with manual transmission speedometer acting funny

about 6 months ago i replaced the speed sensor in the transmission because speedometer was not working, and it worked fine, now it seems like the speedometer is working like a tachometer. if i take it out of gear at stop sign and rev engine speedometer will go up and down. if i am driving down the road it shows 85mps but if i turn heater on or headlights on it does come down a little. any idea what might help fix this problem, thanks

Sounds like you are getting some electrical interference, perhaps from old spark plug wires, routing of the speed sensor wire near other high-amp wire bundles, or a bad EMF filter (in the alternator?). Hopefully others will chime in and help me out with this since I don't have direct experience?