Re: 2005 6.0 misfire w/ no codes
I'd be hooking up a supply fuel pressure gauge ASAP. Low supply pressure will cause intermittent mis-fires and/or reduced power, and will damage injectors quickly.
How are looking at the codes? Most code readers only rear "hard" codes. A high-end (Ford, Snap-on, AutoIngenuity) will also see the "soft" or pending codes which the cheap scanners don't have access to.
I would highly recommend running a cylinder contribution test. That will show you if its a problem with a cylinder or cylinders, or something upstream that is affecting all cylinders. Also watch the FICM voltage. If its dropping below 48v, that's your problem (or at least one of them).
2000 E450 dually V10 wagon