When the a/c or defrost selector is turned to either position. while driving on the hwy. the trans or the engine seems to hesitate and then surge ahead momentarily. and this condition keeps on happening every few seconds until it is switched to the off position. it almost feels like the trans is slipping or locking up momentarily. the local garage kept it for three or four days and trouble shot it. they still don't have a clue what it is or what to do. and i do not have any experience or equipment to check it out.