Best thing to do is get a code reader and see if it pops any codes up. That way you have an idea what to start with. If I recall, it should be an OBD II system.
When it struggles to pick up speed, does this happen randomly? Or does it happen when the engine is cold, starting out in the morning (or when it's sat and cooled down)? Or does it happen when hot? A bit more info is needed.....
'04.... is that the 7.3L or the 6.0L? I forgot what year they made the switch.
If you don't want to get a code reader, start w/ the basics. Change the fuel filter(s) and the air filter. It won't hurt. If those don't work.... then a code reader should be used to help send you the right direction. A computerized engine, especially diesel, could have lots of things that could cause it not to have pep at times. Unlike the old diesels.....
If you don't get a good reply, check out the site listed in sources.... it's a Ford diesel place. Lots of mechanics and owners that may have experienced issues.
