I've found that I shoot my best when I cant my bow to match the natural hand position of my left hand pointing at an object. From this position the arrow falls in line under my dominant right eye, and my misses are usually up and down. Anything other than this cant results in left and right misses.

But I've also found that this level of cant introduces a tendency for arm slap. It's bad enough right now shooting bare arm, so I know it needs correcting before I slap on the cold weather duds.

I know overdrawing is a usual culprit, but that's not the case here. I think it's from the cant bringing the bottom of the string closer to my arm. Is this a common problem with trad bows? How is best countered/corrected?

What level of cant are you speaking of? Is it past maybe 2:30 (o'clock). What is your brace height at? You really want to eliminte as much slap as possible. Your grip, stance (you may have to open it) andbow arm extension can also come into play. If you have not done this before, try griping a door jam as you would your bow, now without moving your grip, move your elbow up by rotating your shoulder.

Lining up the arrow below your pupil is a very important thing. However, you can also acheive this (besides canting the bow) withslight variations of head lean (people usually lean their heads at the same level of cant, but others use variations), and variations of anchor points (not everyones head is exactly the same, you may have to anchor to a different upper tooth).

Yeah, now that I think I figured out what you mean (position of the hour hand between 1 & 2, right?) 2:30 is about right.

I hate leaning my head over, makes me dizzy

I don't think my bow arm is the problem, but I'll play around with stance and lean, etc. and see what I come up with.