1997 Chevy Blazer Sticky Speedometer

I just recently bought a 1997 Chevy ZR2 Blazer and at highway speeds the needle of the speedometer sticks. Between 0-45 mph it works fine, but once you hit 50 it becomes jerky, generally moving in increments of 5, and once the speedometer hits 60, it stays there until I reach about 75mph, and then it moves up to 65. (My GPS displays my current speed so I know how fast I am going.) When decelerating, the needle will stay where it is until I hit about 50 when it will drop to the correct speed and start functioning normally. Any help will be greatly appreciated. Thanks in advance!