[SkidmoreGarage]

Do you know how many pulses per second the signal is? That may help you understand what could be generating a signal, if it is back feeding through the cluster or sensor.

My gut says to follow the suggestions above, if the ground is loose or shared with a noisy source it could be creating a voltage offset that is lowering the threshold for the trigger to where it is picking up noise. If you have access to a voltmeter, you may be able to clamp onto the signal wires and see if anything funky is happening (voltage increase or decrease) when you see the speedo change. I'd also double check the wire insulation for knicks.

If you believe it is fuel pump related (strikes me as an odd source as it is probably a regular dc motor, but could be related to the pump relay if it is opening and closing), you could temporarily move the speed sensor wire and see if it is resolved.