My 85 TD climate control system is similar. I believe that these systems require small adjustments in order to have the time to sense the cabin temp and adjust the mono valve (heat) or expansion vave (A/C). It can be frustrating. The system does respond quickly to a full heat or cooling demand but anything in between takes time. You could easily check the sensing line integrity by removing the passenger side kick panel. Lay on your back and with a good light look up for a foam hose. If this is deteriorated then replace it. Not much fun but that may be the problem. Or as you sad could be the ACC head unit in the dash. E Bay is a source or Fast Lane, lots of on line sources.