A few things could cause this. The first thing you want to do is to determine whether the problem is the cable or the speedometer. To do this, you're going to need to pull the pod again.

You need to make a short cable to drive the speedometer. One way to do this is to get an old cable and cut about four inches out of the end. If you don't have a spare cable, get a finishing nail and file one end square. What you want to do is insert one end of your test cable into the speedometer, and drive it with an electric drill (if the drill is rated at 1200 RPM, it will drive the speedometer to about 70mph). Observe the action of the needle. If it's smooth, the problem is with the cable or transmission end.

If it's the speedometer, remove the face and try the test again. If the spring compresses smoothly, the problem is the spring or needle interfering with the face, which is easily fixed. If not, the drag cup may be impinging on the magnets, or the mechanism may need a clean and lube. Either way, observation is your friend.

If it's downstream, then begin by pulling out the cable and applying a light coat of grease to the lower 2/3 of it's length. Replace the pod and see if you're ok. If not, the speedometer gear may be slipping on the tailshaft, and that's a difficult fix.