Go back to checking for vacuum leaks, especially where manifold bolts to head. Blowing out a small section is not uncommon on turbo cars. Sometimes it is easier to just start with a new gasket and pull the manifold. That's a perfect opportunity to service/replace the crankcase oil separator and replace the O-ring below it.

If no improvement;

Carefully check all turbo hoses from the turbo outlet to the throttle body by examining each FROM THE INSIDE for rips/tears.

Dump the platinum spark plugs and put in standard single ground copper type.

Use regular fuel instead of premium, preferably a brand other than Shell.

If still a problem, you may need to pull the injectors and check for poor spray pattern. Techron helps Chevron more than it cleans troublesome injectors.