My '54 3100 4 speed acted the same way. I replaced my broken cable with just a cable kit that I purchased at NAPA. It was wavy above 45 mph and soon broke. I ordered a whole new cable assembly from LMC or Classic Parts and the speedo acted the same way. Someone suggested putting some vasoline in the speedo and some on the end of the cable. I just put some vasoline on my finger and worked it into the speedo and used a tooth pick to push it in a little further. Also I put some on the end of the cable and reattached. It works like brand new. I can't believe it, something fixed without taking anything apart. The needle is as steady as can be.
