.....Sometimes when I start driving it doesn't register at all, then
wham, it pops up and begins working.....
BTDT Try checking down at the left wheel hub where the end of the
speedometer cable comes through the grease cup.  The end of mine had
been worn round and would slip.  Right now mine is held in place with a
couple of turns of wire behind the little washers that are there for
spacers, pulls it out far enough that the good part of the square end
catches.