My new to me 96 E300D has a persistent P0200 code issue. If I clear the code, I can run about 50-100 miles without it coming back on. Car runs fine. This week I drove about 1200 miles at a near constant 75mph and the light went off by itself. I was using some Diesel Kleen in the tank. Any thoughts on what a persistent p0200 issue means and why it would go away with constant high speed driving or with the Diesel Kleen?