it sounds very similar to the problem that i had some time ago,carbon build up in the turbo stops the variable vanes opening and closing.one can of bg244 cured my problem and at a cost of approx £20 it's worth a try.
greg,take it to the dealer as your car may need the engine rebuild,if you have full service history it will be foc.
prior to my engine being rebuilt my fuel economy tailed off rapidly.
no the ar6200 is to be used as your regular fuel additive to prevent carbon build up and keep the fuel system clean.
the bg 244 is to clean your system of previously accumulated carbon etc.
you should start to feel a difference in performance after about 60 miles or so.
well i have to say that at the end of my test ar 6200 has given me a 7% more miles than the previous tank with millers eco max additive.it provides smooth running and the engines full torque characteristics are maximised.it is excellent value for money at around 65p a tank for substantial economy increase.
503 miles on 58.47 ltrs =12.86 gallons=39.1mpg.