I have the 100-400mm and have not really been happy with it when hand holding. (It's great on a Mono pod) The reason was I didn't like the push/pull and couldn't seem to find the right tension for it.
I was in this part of the Forum and came across a thread in which one member said, that when used this lens he turned the tripod collar to the top of the lens barrel. The difference this makes is fantastic it's so much easier to hold and to zoom in and out.
Not now sure which thread it was or who made the comment but Thank You very much for the tip, you've now made me a very happy 100-400mm owner.