i've got the same problem.. only happens in 2nd and 3rd on fuel boost and when accellerating quickly..
i've had advice from ppl, doesn't sound like the fuel pump, well not for me (Bosch 910) as it seems it's still getting fuel. it might be the spark plugs. i'm gettin mine checked out start of next week, so will let you know

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91 Liberty RS turbo --> 04 Liberty 2.5i --> 99 Forester GT --> 10 Forester XT Premium

check the air flow meter, or air filter might be blocked..

N0 need, he's got Motec

I had the almost similar problem to you, the brant alarm that was fu**ed took it off and never had the problem anymore, i always hate them