S3 diesel misfire

Not sure what it is, but every so often my 2.25d will start to rattle and put out white smoke.
If i ignore it and keep driving it clears after 30sec - 1min.
I noticed it first 2 times when i was very low on diesel so thought it picked up dirt or air in from the tank, but last time I was 3/4 full.
Recently did a 70 mile trip and it did it once in the last 10 miles.

Any ideas?

Thanks
James

ps; will be starting the body job soon.

have your injectors renozzled £20 each and pump done