what would make a transmission jerk a few times, usually between 40 and 50 mph. on 1998 Mercury Mystique

trans. will seem to slip or go into neutral when going into reverse. but only at time. not all the time.could this be electrical? I had a serpentine belt put on a few months ago and they had to lower the motor.could they have pulled something loose? it seemed to work fine before they worked on it.

Asked by for the 1998 Mercury Mystique