Jumpy Crankcase Ventilation Problem

I haven't had the exact same issues, but those do sound like ventilation problems. Is the smoke you're seeing black or more of a bluish color? The color can sometimes tell you more about what is burning. It could even be as simple as a blocked hose causing pressure, like a pinched artery in a body.

The smoke appears to be more bluish. Could a blockage in the crankcase ventilation system cause such a pronounced reduction in power, like a restriction in airflow?

A bluish hue points more towards oil burning, which could be due to that crankcase pressure forcing oil past seals. If the ventilation system is blocked, the engine can't 'breathe' properly. I think it's likely the crankcase ventilation valve is defective and creating too much pressure that affects performance. Given the symptoms, I would take it to a workshop without delay. You could try to diagnose it yourself, but it could get much worse if not properly dealt with.

Your insights were helpful. I'll seek professional evaluation and repair. The repair cost about 360€. Thanks for confirming my suspicions.