I agree with Killie. Does look like a spine problem to me, with maybe an interaction with physical weight as well. If you don't already have them, a spine tester and grain scale are absolutely essential if you're going to shoot wood arrows. There are always variation in spine and weight of wood shafts, often much larger than the specified range ordered. Without those two tools, it's nearly impossible to construct a well-matched set of arrows, or know whether you received what you ordered.