I ended up hanging a speedometer from a 72 binder under the dash until I can find a 5" round one to replace the original. If I had $250 for a new one, I wouldn't still be looking. Since the "hanger" is reading steady, I believe the origial one is worn out. At least I know the cable and tranny gear is working correctly. I had a similar issue with my 66 Newport when is was under 50 degrees. I ended up removing the cable from behind the speedometer, pulling out the inner cable, coating it with cable lube from NAPA, and reinstalling it. No problems since.