Hey,
Kelly and I had this same problem with his car. When the tuner puts the fan in front of the radiator it started to change each run. Unable to get a base line. Mainly effected low RPMs before the boost kicks in.
What we did was take a small sheet of aluminum and bend a shield to mount on the bumper. The shield blocked the air flow from the fan allowing consistent runs. Runs like a Bat out of hell now!! Was effecting his tune on the street before the fix.
ISSUE: Air flow sensors are to close to the filters on the stillen intake. During our runs we used a piece of card board to block that air flow at low RPMs.
TEST: Try a run with the fan off just to get a reading, then do it again. If there the same or close then its the fan blowing air into the air filters. Mainly effects computer before the boost kicks in at the lower RPMs, makes it hard to get a baseline before the tuner starts his runs. .
Hope this helps..