crazy rpm ???

I have an 87 mustang with a 5.0 H.O. and recently the rpm gauge started jumping up and down about 500 rpm. Does not matter if the car is in gear of if I press the clutch in, it still waivers back and forth about 500 rpm. Car drivers just fine, shifts smoothly and seems to idle just fine Even tho the tach. Is freaking out. Any input would be greatly appreciated. Thank you