2006 Subaru Baja odd performance on long trips

I have an '06 Subaru Baja Turbo, 5-speed manual. On two different occasions the car has acted like it is missing, in the way a car would when the tank is nearly empty and the engine is not getting gas. Each time this has happened there has been around 1/3 tank left or more--not near empty and not even near the low fuel warning light coming on.

When the car "misses", it is more abrupt then the "misses" you experience in a car nearly out of gas. There will be four or so quick surges or accelerations/decelerations followed by a few minutes of normal driving then a repeat. The first time this happened I was right by an exit and took it immediately to a mechanic who looked at it but could find nothing wrong. Since I was already stopped, I filled up with gas and continued without further problem.

The second time I was not near a service station and so experience several of the surges until finally the car slowly "wound down"--I know no other way to describe it. The engine remained running, but there was no power. After a minute or two the engine went off. When I was able to fill the tank I preceded with my trip without incident but made sure to fill my tank whenever it got down to between one-half and 1/3 full.

The common denominators in these two events is that my tank was roughly 1/3 full, yet had a solid 5 gallons left in it, and I was several hundred miles into each trip when the problem occurred.

Whether on longer trips or local, I drive until the fuel light comes on--I usually have 40 more miles left in the tank. The problem only occurred on the long trips.

Sorry for the length of my message. This problem is puzzling to me and I just wanted to give you all the information I could to aid in your diagnosis.

Thank you.