I have had similar problems on one of my (series 1) Polski 126's in Cuba.
I replaced the instrument cluster with a second hand one bought on Ebay, because the fuel gauge was faulty.
It is worth checking the fitting of the speedo cable into the back of the instrument cluster. It is pretty fiddly and sometimes the square drive inner does not engage properly with back of the speedo. There seems to be quite a lot of excess end float in this inner cable which allow this.
Now I am sending another instrument cluster out for a second Polski 126, whose speedo has failed, but we are not sure why.
Are you replacing the speedo itself alone in your existing cluster?
If you have the cluster off the car, look at the back where the printed circuit board is.
At the top, right down deep, there are two screws holding the cluster to the housing. It is difficult to see them.
Unscrew these and then the cluster should come clear of the housing when you spring it off the two lugs at the bottom.
The you will have access to all the instruments. Be careful not to damage the indicating needles.
Reassembly is awkward to replace the two screws.
If you do not have a suitable gripping screwdriver, maybe try a dab of grease on the screw head to hold it to the tip of the screwdriver.
