Slight EGR valve and MAP sensor blockage due to constant sooting up on a long run at constant temperature/ fuel rate.... Then sitting in traffic producing more carbon deposits with no real chance to clear them due to the limited time on full throttle through the gears in traffic. Then followed by a constant run again which hardens the carbon build up at a constant temperature again.
Imagine being a heavy smoker, doing a marathon, smoking twenty cigarettes and doing a marathon again with hardened, constricted arteries and a limited lung capacity and you will get the idea of what is happening.
Clean EGR and MAP sensor thoroughly and make sure the fuel filter is also changed a little more regularly
Have been using two stroke oil in the fuel tank by any chance?
If you have then thats a tell tale sign...
Yes it can get rid of most of the carbon build up..... But..... It just moves it on like the Police move on traveller's, it ends up somewhere else that is also undesirable.
The carbon has do go somewhere and not all of it comes out of the exhaust. Plus all of the "oil" doesn't burn at the same temperature as diesel under compression so ends up coating the manifold, turbo and exhaust. Once soot hits the light coating of oil, it sticks, then it has something to build on!
A note to anyone who practices two stroke oil in the fuel tank methods... If it was meant to be used in diesel tanks it would have been included.
Cure for over use of two stroke oil in the fuel tank
100ml into the exhaust manifold of adblue which is used on commercial vehicles (LGV's) to reduce exhaust gasses and carbon deposits once every six month.