Speedo erratic, shows speed when you let off

[JHarms84]

My speedometer has been not working for a while now, its been on and off and really bad when wet out. I replaced the vss in the rear diff and checked all wiring, a part above the axle was worn through, fixed it and still doesnt work. Its very erratic. say i accelerate to 80kms/h (50mph) the speedo will be fluctuating from 15-30 kms/h. but as soon as i let off the pedal and coast. the speedo jumps up to 80kms/h and slowly drops with my speed. (little bit of variation but not much). It jumps up to my real speed every time i let off the gas pedal. If i get back on the gas the speedo drops and fluctuates. Im not sure if could be a bad ground somewhere that gets more effected with moisture but im pretty stumped.

Its a 03 F 250 7.3

Thanks.

[JHarms84]

any ideas??????? It throws a p0503 code too.