One cause of unstable ACC temperature control in a W126 is a deteriorated Aspirator tube. This tube runs behind the glove box liner and pulls air through the temperature sensor (located behind the small grate in front of rearview mirror) With the AC running hold a small piece of tissue paper in front of the grate,it should be drawn against the grate. If not replace the tube with small AC line insulation (black foam tube)available at most home improvement stores.

Bob D.