Haldir,
to figure out any issue in your shooting, remember the fundementals. Take each step and break it down. Do one thing at a time and evaluate your shooting. For example, if you think back tension is the culprit. Do not change anything except add more back tension. If this does not correct the issue then proceed to something different, such as grip, etc. Keep working your way through the fundementals until you solve the problem. It sounds long and slow but hey, we are shooting trad gear. That's the choice we make. all of us can say look at this or check that out, but in the end it all comes down to you. Your form, your arrows, your bow. I find that so much useful info is passed that when you go to put it in practice, you adjust a few things, not one. If you shoot better, what caused it? or vis versa. I gues what I'm trying to get at is make one adjustment at a time and see if it works. If it does you know what caused the problem. If it doesn't, you know that it wasn't the cause.