Vista - Why do my desktop icons change size back and forth

So, I have a new Lenovo ThinkPad T61p with a 15.4" monitor with a resolution
of 1920x1200. It's a lovely machine all around, but that res at that size
makes for a teeny-tiny font and icon size. So, having heard of the nifty new
dpi scaling in Vista (it also shipped with Vista Business x64, I should add,
as so few laptops are shipping with x64 flavors yet!), I set the dpi to 150%
in the custom dpi settings.

This often works quite well. But one of the things I've noticed is that
sometimes the icons on my desktop just randomly switch size between the
default 96dpi size and the 144dpi size. Usually a reboot will return them to
144dpi size. Any ideas why this is happening and how to get it to quit
changing and stay at the 144dpi I've requested?