Based on daughters Golf 130hp GTI diesel my thoughts would be towards the turbo.
Hers was a variable vane turbo that went into limp mode due to vanes sticking, it was freed off with various additives and cleaning products, but the cause was to much low speed town use causing it to gum up and not allow the lever to operate fully in both directions.
Once fixed and every so many tank fulls of fuel adding turbo cleaner cured her problem.
If access is possible you may be able to check the lever movement.