Just a thought, are you using a cable-release (TC or RS 80N3) and accidentally pulling it a bit when you take the shot?

Mirror slap can cause vibrations in the body, FF worse than crop, gets worse with medium format. Mounting via the tripod collar makes the camera stable, but that's the worst place for mirror slap. Putting the camera on the head will get rid of that source of vibration, but bring in others (and maybe the ballhead can't take the angular strain that way). Putting sandbags on either the camera or far end of the lens can help, but unbalances it, and isn't very practical.
Only other solution is even more rugged/rigid ballheads, and tighten everything a lot more.
http://www.manfrotto.com/product/0/2...o_Lens_Support and http://www.manfrotto.com/product/0/3...g_Lens_Support (or equivalents from other brands) are also available for extra cash...