wierd surging problem

[vtjvt]

96 grand am 2.4 liter

The car runs fine, has good pickup and such but when I am going over 40mph and let off the throttle the tach surges (bounces) up and down about 200 rpms, and I can feel the surging in the car, if I give it even the slightest bit of throttle it clears up and runs fine, and blow 40 it is fine. there is no check engine light and no codes, any ideas?????????

[vtjvt]

new info to post, it only does this from 40 to 55 mph, everything else is as smooth as normal.