1993 Jaguar XJ6 torque converter bounce

@ 50 mph car goes into overdrive gear, and feels kinda like the torque converter is locked up because when you let off the accel the cars tranny makes a thumping noise and kinda surges, then is smooth but you hear the engine whine down ( as if in a standard trans at high speed in high gear while decelerating). When you accel again the tranny thumps and surges but is smooth after engaged. I have a theory that the torgue conver is supposed to diengage at O.D, but isn't. PLEASE HELP!