possible misfire on my nissan almera tino 2.2tdi?

The problem was intermitant, occuring at any speed if I allowed the revs to drop to around 2000. The car would start to chug and produce some smoke when I tried to accelerate again and I would have to put the car into nuetral and allow the revs to drop to about 800 before engaging a gear and being able to accelerate again. The problem has become more persistant over the weekend.
Thanks for any help suggestions.