Craig , you mentioned that you get good arrow flight for a while,then it deteriorates.It sounds like to me,that as you tire,your allowing bad habits to creep in on you.You can shorten your practice time, and stop while your still shooting well .As someone else mentioned,have someone watch your form to help find the problem. The problem you are describing sounds an awful lot like one I fight from time to time.It only happens after I've shot way too many arrows. I have to focus on (1) rock solid bow arm,(2) reaching full draw(get it all) and positive anchor,with my draw arm straight in line with the arrow and my elbow feeling as if it is directly in line with my anchor.(3)Then the release is crisp,your string hand remains on your cheek,and the arrow hits the mark,with speed. I hope this makes sense. God Bless