Tons of discussion on this in the archives. This car has a function which compares the RPM of the a/c compressor to the RPM of the engine. If they don't match, the a/c is shut down (until the engine is restarted) on the assumption that something is wrong.

Two common causes are compressor clutch gap and grease/oil on the compressor clutch. The former can be measured with a set of feeler guages, such as are used to check valve clearances on an engine. Clutch gap spec. is 0.50mm. They seem to work OK up to 0.80mm or so, then clutch engagement gets flaky. As for grease/oil, this is an easy DIY. Pickup a spray can of degreaser or brake cleaner and spray it in the compressor clutch gap. Rinse and repeat. That alone has fixed this problem on many an older MB.

- JimY