bogwopits intermittent missfire????

[bogwopit racing]

Hi guys, I have an 07 2.2 td fwd new shape transit, which has an itermitent misfire at around 2200-2800 rpm, it does this untill you put your foot down hard then it clears [in a big cloud of black smoke from the exhaust], also it seems to drop down on to three at tick over when warm, however as soon as you rev it even slightly the missfire dissapears, and as I say does this intermittently, sometimes not returning for days, any ideas?.