I have a 2005 Le Sabre when it gets cold around 40 deg. the speedometer doesn't return to Zero it will stay at highest speed then next drive will go from there to next plus last speed. Reaching say 140 etc. When the weather warms up it will take a few days to return to zero.
sounds like you have a bad stepper motor. you will probably need to have your instrument cluster sent out for repair or you could try to find a shop that will unsolder the speedo stepper and solder in a new one. Not a lot of shops do this repair. It is very simple, there are four connections on the printed circuit board where the stepper motor is soldered. But getting the parts and stocking them makes it for most shops to worry about so they will either order a replacement cluster or send them out to be rebuilt. (less tech time)
if you can solder on a circuit board you can do it yourself. there are four soldered connections for a stepper motor. You unsolder the old stepper and solder in a new one. Plug in the board and cycle the key, that will let the motors cycle and then zero themselves. After you do that, you put the needle back onto stepper motor at the "at rest" position. Takes just a few minutes for each stepper that you change.