99K KW T800 speedo issue

I have a 99 KW T800 with 9NS C-12. The speedo is very bouncy while driving steady. Speedo was replaced a few years ago with the newer style electronic display odometer 100k miles ago. Scan tool shows a steady speed while driving but then will drop 5, 10 or 15mph then pick back up. When the speed drops on the scan tool load % changes and truck acts sluggish, even bobtail. Output speed sensor on trans is fairly new, just a single 2 pin connector. Tone wheel was tight and sensor is backed off 1/8 turn from touching. No abs light on but has active code for LF sensor wrong tire size and inactive codes for atc valve open circuit and wheel sensor left 2nd axle freq too high. Anyone have any diagrams or ideas where to go next?

 

788083 last 6 of the vin. I found a diagram online that says it is for a c12 and that pins 12 and 18 on the chassis ecm connector are for the speed sensor but it shows open circuit with a meter and the sensor has 260 ohms