When driving my 2006 Ford Fiesta continuously for 6 to 7 hours, maintaining an average speed of 100-120km/h, i can feel the engine gradually gives less power, and it shuts down after. There is no temperature warning. I have taken it to the showroom, and they have scanned the engine for error codes, but there was no error code.
I've experienced this once before on a Lancia Y10. The fuel breather was blocked so as the fuel level decreased, a vacuum would form above the fuel in the tank and eventually this would overcome the fuel pump. Upon stopping the car and removing the filler cap, you'd hear an audible "whoosh" as air was sucked back into the tank. This behavior was only evident twice, both on a long motorway run and after over an hour at speed. Using the car on short journeys or at low speeds wouldn't invoke this behavior.
I've had this once in my car also. My catalytic converter made metal rattling sounds so I knew it was time to change it though I did not think much about it. While I was driving to work after 30 minutes my car started losing power. Eventually it stopped and didn't want to start. I got it towed to a mechanic I know. The mechanic took a look and he saw that the catalytic converter was clogged up. After that a new catalytic converter was installed everything worked perfectly fine again.
I would agree that it's something in the fuel system. I have had the same happen, and I replaced the fuel pump. It worked.