Check the positioning of the speed dial. There are a series of steps in the dial, and between 1/10 and 1/25 is a large step. It may be that the dial needs to be positioned slightly one side or another of the marking for the pin and step to be in the right spot. This might be why it is erratic- just hanging on the edge of the step, then dropping off?
You can move a dial until you get the right speed, then make a mark on the dial. The index mark from the factory isn't always in the best spot. It's easier on large view camera lenses, but you can just remember to be offset a bit for 1/10 or so.