Sounds like a fuel/air problem. Either you've got too much air, or not enough fuel.

Too much air comes from leaks in the air ducting on the engine. Look for broken or detoriorated connections.

Too little fuel could come from a faulty fuel pump, a blocked fuel filter, an intermittent fuel pump relay.

When a similar problem happened on my 190E, it was the fuel pump.

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Cheers, Neil