1992 Dodge extended van with 318 v-8 engine
With aircondition selector switch on max vent, while driving on interstate, 65-75 mph...(when the van is cruising on a plain level highway...the aircondition continues to blow out through the vent as selected....however during an incline, the automatic transmission don't shift to maintain cruising speed. the max aircondition will transfer the vent air involuntarily to defrost.. any suggestion on what is causing this.