Select bone?

Hello
At my brand new iMac Intel 10.6.7 21.5 inch 64 bit with cordless mouse strange things happen when I select bones at my object. http://www.apple.com/imac/

Every time I select a bone in Pose mode, the whole object zoom up in size . It did not do it on the old iMac.

Does anyone know how I can avoid this . Look at my video http://vimeo.com/23848731

Thanks in advance
:slight_smile: