111R,
I would suggest graphing all of the results so that a visual examination of the data can be performed. You have to observe the phase pattern and also the tap pattern. As I recall there are 12 know patterns for excitation. Often times 3,9, and 15 will be different (every 6th one) and on...
I would suspect a defective contact. In some cases, a winding resistance test will help identify a problem. I would not expect to see C2H4 in a vacuum type tap changer if properly venting.
I would suspect a bad connection as the power factor decreases with voltage. How did the current and capacitance compare with the test voltage? There was a paper at the 1989 Doble conference that was written about dry type transformers but it covered topics such as: tip-up, tip-down, and flat...
