One thing to check is the sensor located overhead next to the light / sunroof. First try just vacuuming it off, making sure nothing is broken, etc. This can cause erratic fan speed, undercooled and overcooled air. Someone else here knows a way to test it.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro