2000 Blazer - tachometer surge

While maintaining an even speed between 35 - 48 my tach often jumps, for example from 1200 to 1500 repeatedly at approx 38 mph, and you can feel a slight surge in the speed of the vehicle. If you let off the accelerator or accelerate it stops surging. It does not do this at highway speeds. Yr 2000 with 82k miles.