climate control unit

Vehicle Information: Model: 911; Year: 1993; Body Type: Coupe; Engine (type, size, modifications): 3.6 liter; Total Mileage: 208,000;

I currently own a 1993 911 RS America. It has 208,000 miles on it. I purchased it new so I know the mileage is accurate. Over the last year or so, I have had problems with the heating and air conditioning. The heat exchangers have been replaced so I know that is not a problem. They had a rust problem and did need to be replaced. In addition, the air conditioning has been looked at and adjusted by a dealer. It is operating and the compressor is in fine shape. The last time it was worked on at the dealership, they indicated that they hooked the hammer up to it and "found a sporadic code for an in-car temperature sensor, implausible signal. Cleared code and tested system, operating normally. If this concern does return, then the climate control head would need to be replaced, as the in-car temperature sensor is not available by itself."In the course of working on this, the climate control has been sent to a repair place that could supposedly rebuild them. They charged a couple hundred dollars and said it worked fine. Now when I drive it, something is clearly wrong. Sometimes I can get good heat and sometimes I can't. Over the last couple of years, before things really began to fail, the heat seemed to be kind of on and off. If I turned the heat up to the maximum, I got heat. If I left it at a lower level, it did not seem to turn on and off. I suspect the unit is failing. I'm also told a new unit is $1,700.00. Do I have any choice but to buy a new unit or is there something else that could be done? From a practical standpoint, is there any repair to these units? Your thoughts would be appreciated.