Speedometer problems

Sometimes when that happens it's the part of the mechanism that the cable drives on the back of the speedo itself making contact with the bell that moves the needle.

This is the deal, the cable turns all the time when the car is rolling and it turns a part on the back of the speedo.
That part spins next to a magnetic part so it drives the cable via magnetic resistance.
When those two parts come in contact with each other the needle jumps up because the needle is now moved directly by the cable instead of by the resistance of the magnetic part.

I other words, the cable and needle normally never come into direct contact with each other and when they do the needle jumps up way higher than the car is actually going.

This problem is ALWAYS either the cable binding and winding up so when it releases it spins way faster for a second causing the needle to jump up (this one us usually accompanied by some pretty definite noises from the cable)

Or the two magnetic pieces are coming into direct contact with each other (usually doesn't make much noise)

The cable binding is usually a routing an/or cable lubrication problem.
The second one is usually caused by physical wear of the speedo unit (head) and is best solved by sending it to a speedo repair business or replacement.