While maintaining speed or coasting between 30 and 60 mph car hesitates in a sort of jerking fashion. Not audible; you just feel it. Have done major tune-up including wires. Have checked transmission, timing controls and fuel supply and done some minor parts replacement. Problem persists. Does not occur while accelerating. My mechanic is at his wits end. Any suggestions?

it could be many problems really but if u checked transmission and fuel supply and stuff theres only one part of drivetrain left..the rear end. but you would feel that right away when u accelerated so im stupmed.