All or nothing money pit is exactly what i see this as. I spent a couple hours working on this with my full version vag com. The number of intermitent sensors and other parameters was endless. There were 3 main issues that i picked out as clear a day being a shot maf, bad thermostat and possibly coolant temp sensor, and intermitent signal to the n75 boost control valve. Each one of these problems could cause poor running conditions, let alone all of them put back together. Even as farmiliar as i am with 1.8ts and vag com diagnostics there is no way i could have proceded with anymore troubleshooting untill the above problems are repaired. That is not saying that fixing those problems will solve the issue. all the sensors on the car are my eyes and ears on what the car i doing, take a couple of those away and the scales on my computer might as well be an etch-a-sketch.