I just went through TDI hell that you seem to be entering. Over the last year I have had to clean the intake, replaced the N75, replaced the VNT valve, replaced all the vacuum hoses, replaced the MAF and air filter twice. Just this afternoon I finished with the fuel filter thinking that was clogged. It was dirty, but not clogged. while my son slowly increased rpm's I stood there watching, thinking of a way to run the car into the river when I noticed when he took it to 2400rpm's just as it would start jugging out and blowing white smoke out of the air filter box and the end of the exhaust, the anti-shudder valve was only partially open.... when I held it all the way open the car ran GREAT! I unplugged the vacuum line, stuck a stick in the line and took it for a rip down the freeway; it ran better than it ever has! I put my foot in it and as my wife and I talked I looked down and was bumping up against 100mph, backed way off the throttle, and it ran smooth all the way home.... now I gotta see if the anti-shutter valve is vital... check the position of your anti-shutter valve when it starts chugging out. push it open by hand and see if the engine smooths out..