teleconerter

Hi! I am using the 1,7x Nikon teleconverter with a 70-200mm/f2.8 and paradoxically the photos are blurred at a short focal length (e.g. 119mm) at a big aperture until I stop down up to f10 or so - but they are not when at 340mm under the same settings - so it is not a matter of depth of field or...