Smoking diesel engine on motorway driving Citreon Picasso?

I have noticed that my 2004 Picasso has started smoking when motorway driving and seems unresponsive when reaching 70mph (i.e. upto 80 mph) smoke is dark in colour and is inconsistent in amount i.e. sometimes you can hgardly see it othertimes its very visible. It also seems more prominent going uphill as is the unresponsiveness of the engine. Doesnt blow smoke when idling but if heavy revved when stationary dark brown some comes out