more than likely you have an air leak.in other words there is air gettin in after the the fuel metering valve plate which you can see when you remove the air filter.
check all the turbo hoses for loose clips and any air pipes that are on the inlet manafold.make sure the oil filler cap is a good fit, if you have a dump valve fitted then remove it and blank the hole just in case this is the problem.
see how you get on