I had the exact same problem as you discribed.... what it was,was the vacuum senser ( think thats what you call it) for the turbo. All I can tell you where it's located.. Looking at the engine from front of car, behind the airbox under the bulkhead you will see black or grey valve with tubes coming from it. That was my problem,,,, the car would loose power, rev counter drop to zero, engine sounded like a bag of spanners and engine/coolant light came on. Once renewed NO MORE PROBLEMS, but it cost me about £70 for the part if I remember...
Hope this helps
Andy, August 2010