My 98 Outback runs fine for the first 10 minutes as it warms up. After that, I have the following problem: intermittently, when I lift my foot off the accelerator peddle, the car seems to lose power. When I then press down on the accelerator, the car hesitates for half a second, then bucks 2 or three times before it begins to accelerate. This happens in particular when I am going down a hill and the wheels are turning more than the rpm. During all of this, the clutch is fully engaged. It feels as if the car misfires when I lift off the accelerator, and then****** hard to accelerate. I have had the ignition wires and coils replaced, and recently the engine rebuilt. It made no difference to the bucking. Can you give me a idea of what the likely cause is? Thanks.