I have a 2001 F 250 V-10. I pull a popup camper that weighs between 3 and 4K. When I am towing for about 50 miles or so at interstate speeds, the truck will lose about 500 RPM when I let off of the gas pedal like when coming down a hill or slowing to pull off the highway. This will only last a few seconds and then it will jump back up to normal engine speed. If I hold the pedal steady, it won't do it. If I am not pulling a load, it won't do it. Recently, it did it a low speeds when pulling up to a red light. I have been told that it might be the boost valve or it might be the throttle position sensor. I don't want to replace parts randomly. I've been told to put a scan tool on it ans test drive it, but that seems impractical. I don't have access to a scan tool and the problem is so specifically intermitant that I can't be sure when it will happen. I am not getting any "check engine" warnings. Any suggestion?