There's no tightening that can be done on a belt design that uses a tensioner. It sounds like the driving you do with the a/c on is the design limit of the belt to where it slips a bit during shifts.

The fact that it only happened twice during your Arizona summer tells me your system is designed properly, being on that 'fine edge' without being overly tight.

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95 E320 Cabriolet, 169K