Spine issues generally show up as lefts and rights. At extreme high speeds, even 10 grains can make a pretty big diff, but 6" may be a bit extreme.

One way to rule out the weight as an issue is to add 10 grains of weight to the point end of the light arrows and see what happens.

I would remove the point and inserts and make sure they are all in perfectly straight.

Also check for worn out/too tight nocks when you rotate them.