When it gets very cold outside sometimes the speedtable will get all funky and lose its timing causing it to play 'catchup'
Possibly a bad vehicle speed sensor (VSS). This sensor is usually located in the side of the transmission.
Broken cable?
I think you need a speed sensor
Sounds like you need a speed sonsor
There may be several reasons whey the digital speedometer might not work. The first is that the LED light may have burned out. The second is that the fuse may have gone out. It also might have a bad speedometer cable.
If your speedometer isn't working then that gear needs replaced before it'll shift
i
The speedometer is cable driven.
A 1991 dodge dynasty speedometer bouncing could be a sticking speedometer cable, or a bad drive gear. Check the cable first, and lubricate it to see if the problem stops.
The 1991 Geo speedometer cable is held in place with a swivel nut on each end of the cable. Turn the swivel nut to the left to remove the speedometer cable. Reverse the process to install the new speedometer cable.
It shouldn't have the cable, but first, are you sure its the speedometer and not something else... you need to listen carefully to where it might be coming from, might be wheel bareing, tire, could be serveral things so just eliminate all the possiblities so you will know what to do and how to fix it.
Jimmy Crapnell died in 1991.