Jumping Speedometer

I have a 95 Sable. When driving all of a sudden the needle jumps 20mph. It will go jump wildly from the speed I am going, 30 - 50 - 40 - 60 then back to 30. What can I do to fix this myself. The car has 99900 miles on it so do not want to dump alot of money into it.

Sounds like the speed sensor in the auto. transmission needs to be replaced