1998 Subaru Outback • 230,000 miles

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.
October 7, 2011.

Possibly EGR valve or throttle position sensor.

Oct 7, 2011.
I replaced the TPS and the EGR checked out as working fine. The problem remains, it feels like it loses power when you lift off the gas peddle and when you press the accelerator, there is a half-second pause and then it bucks before it accelerates. I noticed for the first time yesterday that it is beginning to happen when I am in overdrive with the cruise control on at freeway speeds, it loses power and then bucks lightly as the cruise control eases off and then back on the gas. Any more suggestions?

Oct 22, 2011.
Test post

Oct 23, 2011.