[guye]

Me too. Same issue with my Speedhut, but only once so far.

I have my speedo wired in parallel with the ECU. While the VSS from the 'box issues ac current the ECU sends 5v dc back and I suspect might be the cause? I'm thinking of putting a diode on the line to block the 5v from the speedo, cause it only needs to reach the sensor anyway.

Also, I find this tidbit on Classic Intruments website about trouble shooting erratic speedos:

"To check if the erratic speedometer problem is caused by ignition noise (requiring shielding) or by the transmission, drive the vehicle until the speedometer becomes erratic, then shift into neutral and turn off the engine. Turn the key back on so that the speedometer has power going to it. If the speedometer is steady without the engine running, then the problem probably will be solved by shielding the speedometer. If the speedometer remains erratic without the engine running, the problem is in the transmission and might be solved by using the SN74 to buffer the signal."