break in continuity when test gas sending unit

Hi, when measuring ohms of the fuel sending unit of my 64 Dart, the ohm readings are a correct 15 and 78 at the far ends of the gauge, but I have to jiggle the float arm to get a steady reading, otherwise the continuity breaks. It's especially hard to get a middle range , or half tank reading. Ohmmeter leads are clamped to clean metal with Jr. Visegrips....any ideas??? Thz