1987 Sedan Cadillac Devile Speedometer

I have a 1987 Sedan Cadillac Devile and my speedometer works on and off and depends on how a accelorate and whatnot on wether it works or not. I was wondering if there was a simple fix other than a new digital display because thats what my mechanic said was the problem and i dont think it is. Also my car dont level properly and the motor and air lines work and dont leak any suggestions?