the problem might be related to the belt itself , in my 87 300d if theres slippage then the compressor cut off relay will cut out the compressor if the difference between eng speed and compressor speed is more than 30% for a certain amount of time (in seconds, dont remember how long) yours is probably the same or similar, this is done to protect the eng in case the compressor locks up. You mentioned the problem is intermitent so I dont think the comp is locked up otherwise you'll see the alt light come on the dash and lose power steering since that belt will definitely self destruct or jump the pulley if the compressor did lock up

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87 300d Turbo 241K with original #14 head (blue)
87 300d Turbo 198K will run great again someday (silver)
84 300sd sold ( buyer drove it to Paraguay )