If I am reading your post right, your friend brought his gun along and got the same results......does this mean that you both shot a group in one place, and the next groups with each gun was off, but BOTH off in the same direction?
If that is the case it is the conditions, plain and simple. What are you using for wind flags....how many, where are they placed????
As far as your chrono, if the light changes a bit, and you are not using a good chrono, or are not using IR screnes, a 40 fps change is common (at least from what I have seen).
Keep plugin away........
It's fun, isn't it??!!!!!