1970 Chevy El Camino speedometor

i have a 1970 el camino. car runs great only one problem. when im driving of the parking lot not going more then 5 or 10 mph, the speedometor jumps from 0mph to all the way to the max mph. might it be that i had some work done to the airbags? "" air bags as in the suspension that makes the car jump"" it manages to only do that as im driing slow but sometimes as im driving fast to. im trying to sell the car so any help would be appreciated since i cant sell it like that, thanks a million