there are several causes of your problem. I would check to see that the vacuum hose to the modulator is connected and has the same vacuum reading as the manifold (with the engine idling). next replace the vacuum modulator. if that does not work, drain and flush the transmission. be sure to replace as much of the fluid as possible. not just 2 -4 quarts. if this doesn't work, many of the major transmission shops will diagnose the problem free.
