Speedometer jumping around when accelerating
I have a 2000 Mazda B3000 cabplus pickup and recently I noticed when I am on the highway and I am accelerating around 60 and over the speedometer jumps all over the place (sometimes 50, sometimes 0, sometimes 60) it never reaches the proper speed that I am going. Once I take my foot off the gas the needle then rises to the proper speed limit (say 70/75) and slowly decreases normally until I put my foot on the gas and accelerate at a higher speed. It will then jump around all over the place again. What can be causing this issue?
Thanks for your help in advance