moving multiple stacks - slight stutter

[John Craig]

I saw rocketdock mentioned recently a couple of times and it's reminded 
me of a small problem (more of an annoyance) I encountered.  I wrote a 
dock using rev and used individual stacks for each icon to get the 
transparency effect.  The stacks are lined up next to each other and 
moved/resized as the mouse moves over them.  It's not totally obvious 
from the video, but the 5th and 7th icons (stacks) move with a slight 
stutter compared to all the other ones.  It always seems to be positions 
5 and 7, even when switching the order of the icons around.  Any ideas 
why this would happen? - each stack is a clone of an existing template 
stack, so they all have identical properties.  Any tips / ideas appreciated.

JC

short video;
http://splash21.on-rev.com/temp/dock.html