OK Avant, this thread is better in Technical. I hope somebody better qualified than I am is able to give a solution

I've suggested a very mild version of the Italian tuneup because it can be a bit hazardous on our crowded roads. The proper version is to chose a long straight dual carriageway early in the morning when no-one else is around and start to accelerate like a Bat out of H***, running through all the gears until you approach the red line on the rev counter. That used to be called a 'lekker burn-up' in Central Africa where I learned it. That tends to clear all the carbon that is settled in the cylinders. Oh, a refinement is to have a friend drive behind you, at normal speeds of course, in case you find the cause is something going wrong and you terminate it.