ORIGINAL: Kreag
The roll isn't that slow, in fact its quite fast... almost snappy. Sooo many people at the field have looked, prodded, lined up, and generally oggled at the wings.. they are very square and true.
That sure sounds like one wing is stalling in the loop. Especially since you mentioned in your earlier post that it only happens when you're doing tighter loops.
Try some straight ahead stalls and see if you can detect a definite tendency for it to drop to one specific side. If it does then you'll want to study the shape of the wing for any differeences from one side to the other. For example a leading edge that is improperly or differently shaped from one side to the other can easily induce an earlier stall on one side compared to the other.
You may need to use some sort of templating techniques here. THis is a time where small differences cause big results. Something like 1/8 plumbing solder or #14 copper electrical wire carefully bent around one wing's leading edge and then used as a template to compare to the other side can work well. 1/32's count for the world here and even 1/64's can make a big difference. The first 1/5 of the airfoil on the "upper" surface, depending on if you're pulling positive or pushing negative G is the most critical portion of the airfoil in terms of maintaining similar airflow. And the first 10%, basically the leading edge, is even more critical.