I've had the same problem, transmitt on the KX155 and  the S-Tec 50 altitude pulls the A/C into a climb.  Talking with a S-Tec tech rep at Sun-N-Fun,  he said the Altiude Transducers are very easily infuenced by RF, He said to seperate the radio and transducer cables where you can and wrap the transducer in Aluminum Foil and ground the foil to the cable ground mesh/wire..      Haven't tried it yet since controller and alt transducer are at Mineral Wells. S-Tec for repair..  Had a bad Transducer, Scott Kruger's spare transducer is there for check out..      More in this continuing saga as this soap opers unfolds.  Don Skeele  2235/320