Zak, the inconsistent shooting and poor grouping you describe - from your two 'good' arrows - is a perfect example of what one would expect with underspined arrows. IMO, you were underspined at the point where you bought those arrows, especially intending to shoot 100 gn fixed blade heads. You definitely need 5575's.

For future reference, when you look at the spine chart for carbon arrows and find you're right at the upper end of one shaft's rating... where another 25 grains of tip weight, another 5 pounds of draw weight, or another inch of arrow length will put you in the next size... always, ALWAYS, jump up to the next size. That goes double when you intend to shoot fixed blade heads.