I am wondering if you might have something cross-threaded where the breakage occurs. If cross-threaded, those threads would have extra tension, which could lead to breakage.
If this were the case, you might notice on the weaving, but for me, I am terrible at seeing the problems.
Where did you find your pattern? Its so cute - like a } curly bracket, with a perfect little black dot.
I recently wove up a Yak-Geelong warp, and had trouble with breakage. The fiber passed the snap test (snap a length to see if you can easily get it to break), but I supect that the strength was not uniform along its entire length.
and for those on the east coast, we talked to college admin today and set dates to present the Olds College level one master weaving class at the Gaelic College in St. Ann's (Cape Breton), NS next June 5-9...