Audi A4 1.9tdi lack of turbo boost

[DRCDG]

I have a 1998 A4 1.9tdi and the turbo seems not to be working at present. The Airmass flow meter was replaced a few years ago and this fault has happened before (15months ago), lasted for about a week, then rectified itself. Basically the car power is very flat but otherwise goes ok 9no funny noises etc) The only other clue is that when the engine is hot and you restart it, you get the revs flying up to 2500-3000 revs before dropping back down to idle, and a plume of diesel out the back. Also, a slight tinking noise from the area around the turbo when engine turned off. I have spoken to audi and they are clueless.

Can anyone advise on the course of action? Hopefully without costing thousands!