I've got an 88 GT I swapped a 327 SBC in with an Archie kit. Got everything working properly with exception to the speedometer. Speedometer will jump randomly up to 20 - 40 MPH and back down with the car stationary. Anybody ever seen this?
It tends to do it while reving the engine more so than just idling. Haven't been able to check if the speedometer functions while driving yet. I've tried a new speedometer sensor and did same thing. I did delete the cruise control, cut wires and shrink wrapped the loose ends. I did the same thing on my 87 GT that I previously had this Archie kit on, and all worked okay. I searched the forum and couldn't find the same symptoms that I have here. Next step I was going to check was to swap out speedometer gauge with a known good spare that I have. Any input appreciated.
It sounds like you've got either a faulty connection to the speedo gauge, a faulty gauge, or perhaps some electromagnetic interference being induced into the purple and yellow signal wires from the sensor, perhaps by running the sensor wires too close to something like ignition system components.