Had this problem for several years,my wallet was frightened to take it to the garage ( :-) ), problem was solved when in garage for another job ( never even mentioned it,they just fixed it,no charge!).
What was it...... small hole in plastic vacuum pipe where it goes under turbo trunking,repaired with tape...... I've now got a new car!!!!!
Vacuum problems are commonplace apparently,check that pipework!
Hope this helps anybody having the same problem.
Cheers Pete
pete mortlock. staffs, July 2014