Just to throw another idea out there.... On my '87 300D, the clutch was going out on the a/c compressor. There is a sensor on it that keeps track of the rpm and if it is significantly off of what it should be in relation to the engine rpm, ie slipping or locking up, power is cut off to it. When you turn the key off and restart, it resets it. This is done to keep from burning up the belt in case it is locking up. Check the clearance on the a/c clutch. Also, check to see if you have oil getting on it causing it to slip.

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05 SL600 75K
87 MB 560SL 119K
87 MB 300D 180K
83 MB 240D 110K
64 1/2 Mustang
74 Monte Carlo 32K
75 Fiat 124 Spyder
16 Equinox
15 2500HD Silverado