I found out that many themes have a problem with sidebar position if I split longer post with "insert more tag". I thought it could be big picture or some copied codes, but I can reproduce it now. If I split post with more tag sidebar is at bottom of a page, after removing more tag sidebar is on right place (I checked with Firefox and IE).
Is there any solution for this problem. That make no sense if it not possible to use split function for longer posts.