Everthing I've read states that if impact is consistently left, the arrow is too heavily spined, and vice versa for right impact. If I was you, i'd try a 100 grain head, and retest. Remember, you will increase spine by shortening the shaft or using a lighter head. The spine values on shafts are just a starting point. You have to experiment to get good arrow flight. Also, check that your arrow nocks are not too tight on the string. if you have to really push them on hard, they're too tight. Also, FF strings usually requires heavier spine. Good luck, and keep trying different things, until you get what you want.