[Sailfish]

I assume you have a Smith's speedometer---------if not, disregard this note. I've experienced the same issue and found it to be the reverser on the back of the speedometer. A piece of the reverser worked itself loose by backing out of its normal position. I ordered a replacement reverser and it appears to be a more robust design (it is "staked" in place preventing movement). I installed the new reverser and everything works great. I ordered the replacement from Nisongers and it cost around $90.