I have a 2000 Montana with 150k on it. I've asked around a bit and no one has heard of this problem: When the air conditioning has been on for 20 minutes or so, the transmission starts slipping. It "clunks" from 1st to 2nd, 2nd to 3rd, and so on, every shift. The problem does not go away after the air is turned off. Can anybody please help me find a solution to this problem?