Fiat Croma Boost Pressure Sensor Issue

Those symptoms do sound like a boost issue. The jerking, hissing, whistling, and power loss all point that way. The stalling at idle could also be related, as the engine management struggles to compensate for the incorrect air/fuel mixture. Has the engine light just started appearing, or have you already checked the error codes? Also, does the whistling/hissing sound seem to be coming from a particular area of the engine bay, such as around the turbo or intake manifold?

I scanned the car and the error code says 'boost pressure sensor'. I agree, it must be the boost pressure sensor. The hissing sound seems to originate from the front of the engine, possibly near the turbo. Is it possible to check the cable or plug connection myself or do I need special tools?

Ah, the 'boost pressure sensor' error code does strongly suggest a problem with that sensor or its wiring. Since the hissing is coming from the front of the engine, near the turbo, it's very plausible that the cable or plug connection is the culprit. While you could visually inspect the connection, cleaning electrical connections, especially on sensitive sensors, can be tricky. If you're not entirely comfortable, I'd advise taking it to a trusted workshop to have them diagnose and repair it properly. This will minimize the risk of causing further damage or introducing new problems. They have the tools to test for any leaking and the proper replacement part.

I took it to a local workshop, as you suggested. They confirmed it was indeed a defective cable connection to the boost pressure sensor. They replaced the connector, and now the car runs perfectly. The total repair cost was 95€, which seems reasonable. Thanks for pointing me in the right direction!