I have just bought an Audi A6 Avant 1.9tdi on a Y 2001 and the engine has got an annoying judder when the thermostat starts to open? It seems to loose power and the more pedal you give it the car vibrates violently. This only lasts for about 10 mins but sometimes I can stop it by turning the ignition off and back on? Or left to tick over for a short wile. Then the problem seems to go away and the car runs sweet?

i have replaced the coolant temp sensor cleaned the EGR and MAP sensor but the problem won't go away? Any help would be appreciated