Peugeot 508 Engine Performance Issues

Your symptoms certainly point to a problem within the intake system, possibly the intake manifold flap control as you suspect. The hissing sound could indicate an air leak downstream of the turbocharger, potentially around the intake manifold area. Has the engine warning light illuminated? Have you had any diagnostic checks performed to retrieve any error codes?

Yes, the engine warning light is on, but I haven't had any diagnostic checks yet. If the intake manifold flap control switch is defective, would this cause all the symptoms I am experiencing? Could it also increase fuel consumption significantly?

A defective intake manifold flap control switch could very well be the root cause. This switch governs the airflow into the engine, and if it's malfunctioning, it can lead to improper air-fuel mixtures, causing reduced power, rough running, and increased fuel consumption. The jerking and hissing could be related to the engine struggling to maintain the correct airflow. Given the engine warning light and the consistency of your symptoms with a faulty switch, I would advise a visit to a qualified workshop for a thorough diagnostic and potential replacement. This could cost you approximately 465€.

I appreciate your thorough explanation and advice. I will schedule an appointment with a trusted workshop for a diagnostic check and repair as recommended. It sounds like a faulty intake manifold flap control switch is indeed the most probable cause. I'll let you know how it goes.