Many problems, especially hard starts, on the 6.0L's can be attributed to a poorly functioning FICM. The FICM basically controls the firing of the injectors, and when (and we mean WHEN) the board begins to fail and lose voltage (below factory 48 volts), it will cause various run-time issues, hard starts, excessive smoke, and the list goes on.
The 6.0L Powerstroke relies heavily on consistent voltage out of the fuel injection control module to properly fire the injectors. The original FICM frequently fails and/or delivers less than adequate voltage, causing hard starts, rough idle, poor fuel economy, and overall less than satisfactory performance. If your FICM is in need of replacement, check out this quality remanufactured unit from Alliant.