95 Lesabre A/C Heater Problem

I have a 95 Limited Lesabre. When the air is turned on only very hot air or cold air will come out of the vents depending on whether the temp is set to hot or cold and no temps inbetween. The digital climate control display works fine and the local shop thinks it might be the heater a/c control. Does anyone have any experience with this issue? Thanks.