Swamper - you might pick up a shaft or two in the 340 series and try them - with and without fletch. See if that clears up the problem. Not real technical of a solution,but it should tell you if the arrow is the problem.

Looking at the charts, and considering the extreme force curve of the bow, it very well be a weak arrow at that length.

I have some 340 series arrows that I left full length (31"), and they are spined pretty well for my 61 lb. bows. Too bad you can't cut those FMJ shafts backsome. I think you would need to trim them about 1.5"to get rid of the HIT that's in there now.