It sounds like a paddle-wheel going bad (I'm assuming your PP isn't GPS), since you're seeing the problem in both the speedo and on the PP display (paddle could be getting hung up (can see if it spins easily by flicking it with your finger or shooting compressed air or the hose at it on the trailer)). If your PP has an RPM mode, you can try that, and if that holds steady, you've at least isolated your problem to the paddlewheel itself or the wiring.
First thing I'd do would be to disconnect any plugs/connections, check for corrosion or loose connections, and put everything back together with some dielectric in the plugs and test again. If that doesn't clear it up, then look into swapping out the paddlewheel assembly which is easy to do.
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