1997 Subaru Impreza - Loss of Power when accelerating

I have a 1997 subaru impreza. When accelerating to overtake my car seem to hesitate and the go forward, sometimes the engine seems to stay at the same rev and then seems to free up and go well. This is an intermittent problem and very dangerous when I overtake at high speed, as the impreza on several occasions hesitates and then surges forward. I have taken the car to a subaru dealership and they have put all sorts of diagnostic equipment on it and says it is fine. Thry have tried it out on the road and found no fault.(Like I said, it is intermittent). I wonder if anyone else has had this problem. The plugs have been changed, air filter cleaned and there is now a new fuel filter on it, but alas no change. Please, please is there anyone to advise. The engine management has been check and everything is fine.

Liz