Black smoke what else!

Driving along the motorway 60mph in my 96 300tdi in my rear veiw mirror I see a cloud of black smoke this will only last for about 2 secs then clear

this may happen again after about 30mins or may not do it again that day

Anyone any Ideas ?

I have heard a lot about the EGR is there a link anywhere to help me understand what it is and the benifits if any about disconnecting it
Thanks
Sheriffofnottingham