Skoda Octavia Diesel Engine Problem

Whoa, that sounds rough! Reduced power and jerking is never fun. The hissing noise makes me think it's definitely an airflow control issue, likely around the intake manifold flap. Basically, that flap is supposed to regulate the amount of air going into the engine, especially at lower speeds. If the intake manifold flap control is faulty, it can't control the airflow properly, hence the symptoms you're experiencing. Is the engine light on? Have you tried scanning for any error codes?

Thanks for the reply. Yeah, the engine light is on. I've never scanned for error codes, what do you mean by that and how do I do it?

OK, if the engine light's on, that pretty much confirms something's up with the engine management system. By scanning for error codes, I mean using an OBD2 scanner to read the fault codes stored in the car's computer. This would pinpoint the exact problem, but if the assessment is right, then it's a defective intake manifold flap control that causes the airflow control not to work properly which leads to the reduced power. Given all the symptoms, it does sound like a faulty intake manifold flap or its control system. I'd suggest taking it to a decent mechanic to get it properly diagnosed and repaired to be honest.

Cheers for the help. Ended up taking it to a workshop, and you were spot on. They replaced the intake manifold flap control. Cost me 465€ all in, but it's running sweet now. No more hissing or jerking!