It could be the speedo itself or one of the speed sensors are acting up on the transmission and it sounds like the output speed sensor is causing this.Their not hard to change really and they plug in , could be the sensor needs to be removed cleaned and re-installed or the plug on it is faulty or rusty etc or the sensor itself is faulty.The labour shouldn't be more then really 30 to 45 mins tops on replacing that sensor but if you can work on vehicles yourself then it's really only a few more mins to do it yourself.
Hope that helps and best of luck.By the way a good parts store or service depo can tell you where the sensor is located and they sell for about 40 bucks or so.Also a good do it yourself auto wreckers may have a speedo like yours cheap or even some speedo sensors you can take off yourself too.Also check the wires going to the speedo too.