This might be the problem,, but my zafira dti on a 51 plate had this problem. The rev counter would drop to zero, the spanner light would come on, it would rev up and down and I couldnt get above 50 mph. i was told the car was in limp home mode. I took it to a garage ( I trusted ). Idont know what its called but,,, It was a vacuum valve. Open your bonnet on the left hand side right behind the air filter box unber the bulkhead fixed onto the wheel arch there are two plastic vacuum valves either grey or black. One of those had brokedown. Once replaced the car went like s*** off a shovel. The valve had something to do with the turbo.
hope this helps
Andy
April 2010
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