My EGR stuck and I had lots of black smoke plus the huge power loss sometimes. I don't know why the black smoke appeared but it could be excess fuel getting in via the EGR which was jammed open. Yours may be stuck in a partially open position which might explain why you aren't getting the normal power loss.
If you're comfortable wielding spanners try to blank the EGR off or even give it a thorough clean - search the forums for details of where to find the EGR and how to remove and clean it. It took me a bit less than 2 hours it wouldn't be hard to be quicker.
If you blank off the valve you will get a Motor Control System Failure message, at least on the later cars, don't worry about it it's the EGR being blanked.
Give a man a fire and he's warm for the evening.
Set a man on fire and he's warm for the rest of his life.