This is due to Origin points not correctly set for all your animations. For instance, the Idle anim has Origin set to bottom center, but the run right is set to center.
Also, some of the collision masks are a bit off :) Just use bounding box for now.
yeah I changed them to see if it would have any effect but unfortunately this isn't the case. It doesn't seem to mind where I put the origin even if I put it in the same place as the running animation (in which my character doesn't sink in the ground). Is this because of the collision masks (don't even know what they are, same goes for bounding box <img src="smileys/smiley9.gif" border="0" align="middle" /> )?
I wish there was a help document <img src="smileys/smiley8.gif" border="0" align="middle" />
EDIT: ah, I found the collision polygon. this fixes the problem!