I hope this isn´t too far off as to be redundant, but I had a similar problem with my Nikkor 4.5/300IFED. It was always difficult to control for the same reason that you have: so sensitive that just releasing the focus ring was enough to shift the thing away from where it should have been.
I did contact Nikon and they told me that it could be fixed by sending it back to them, and that it would then match the other lenses for feel. I never did that as I was about to embark on a terrible gear change to Pentax 67. Anyway, they did say it wasn´t serious and was fixable.
I don´t know if this applies to a/f lenses or not, but maybe an official response from the maker would be a good step in the right direction. I´m sure Canon knows the answer.
Good luck
Rob C