Seat Alhambra Diesel Engine Issues

I had a similar problem with my Alhambra, though it took me ages to figure it out. Check the vacuum hoses connected to the turbo bypass valve. A leak there can cause a pressure drop and throw off the boost pressure. Is the compressor surge very noticeable?

Interesting. Where exactly is that bypass valve located? And what do you mean by compressor surge? Is that the whooshing sound I sometimes hear?

The bypass valve is near the turbo, you should be able to spot it by following the vacuum lines. Compressor surge is when the compressed air from the turbo flows back out because the bypass valve isn't opening properly, yeah, it can sound like a whoosh. It's worth getting it checked at a workshop; they'll be able to diagnose it properly.

Big thanks for the help! Took it to a mechanic and they confirmed it was a faulty vacuum switching valve. New valve fitted and all sorted for 140€. Engine performance is back, turbo lag is gone and the check engine light is off. Really appreciate the advice!