1999 peterbilt 379 speedometer malfunction, check engine light on

Hi recently I started having trouble with my speed ometer and was hoping someone could help me out,

I drive a 1999 peterbilt with cummins n14 engine few weeks ago the speed ometer started to make variations went from 70 mph to 30 mph or 60mph to 20mph etc then the check engine light went on , the cruise control stopped working as well, I assumed the speed sensor went out so I replaced both they are mounted on a 13 speed transmission everything worked fine for about 2 hours then the problem started all over again the speedometer stays all the way down and after a few miles the check engine light goes on again , cruise control stops working as well ,when I'm running the speed ometer stays down but when I release the throttle the pointer goes to the exact speed I'm driving at, when I step on the throttle it goes back down again I have checked the harness that go into the speed sensors and both seem to send signal just fine , I have been working on it for several hours now and still no luck fixing it any help would be greatly appreciated

 

Thanks a lot for the suggestion I will add a ground cable to both

 

I replaced the pedal position sensor a year ago or so but I'll replace it as well doesn't hurt to try, thanks

 

actually no I just replaced the sensor never taught about the pedal it self

 

thanks a lot for your advise I checked my yoke and it was 1/4 of an inch loose I replaced the nut with a new one for safety tighten it well and is working good as always thanks for your tip