Mine would do the same thing when my speedo was being erratic. It wasn't often, but it would lose all power just for a split second and it was always when the speedo needle was well over 100 mph. I havent experienced it since torquing the nut. FWIW, I put a very slight amount of RTV on the shaft when I was retorquing the nut, so far the speedo is still reading true.
Again, similar issue. 0-30 was usually fine, but once I started going faster is when it would become erratic. Usually, though, when I was at 65-70, the needle would just drop and hang around 20 mph. Also, FWIW, when I had my speedo unhooked while waiting to put on the new JB connector, I was throwing a code for VSS speedometer (or something along those lines). My jeep would actually stall out at slow speeds with it all unhooked.