1972 Scamp Intermitent Speedo Problem

Under load, intermittently the speedometer starts screaming like a banshee and the needle fluctuates between 40 and 90 mph. This is when I'm going 25mph. If i read up under the dash (yes while driving) and push or pull on the speedo cable it sill sometimes stop. It seems like the speedo gear may be worn. Anyone out there have experience with this? I want to fix it but i want to have all the parts ready before I tear the dash apart.