You are 100% correct Rev!! I mark all my arrows with a number, then and or when I begin to experience any abnormal flights, I note this and put them aside and find out why. Perfect example. I just bought a dz of GT Velocity XT 400's. Cut them. Weighed them. Found 2 that were 10grains heavier then the others. Go figure?!?!?!? :confused0024:
I then weighed the inserts and all was good. My GrimReapers to be used. All good. I took those 2 and cut off an 1 inch and was close enought to put me with in 3-5grains of all total. 345grns average total with 100 grn BH on my electronic RCBS powder scale.

Drop away rests you shoot the arrow cock vane up. I agree your form could slip a little as you get more tired. When you have an arrow that doesn't seem to shoot right, mark it and shoot it again! Mark it so you don't know your shooting that same arrow until you pull it from the target. This will give you the most unbias results. If you know which arrow shot bad before you shoot it again you will tend to make sure your form is perfect. If it continues to shoot bad you'll know it is the arrow and not just you.

number your arrows , if its 1 or 2 of them , the same ones doing it all the time , it could be many things , fletching clearance , spine , and nocks , are 3 common problems , nocks and fletching can be replaced , if its a spine issue , ditch the inconsistant ones and get new .