Custom Icons Reverting to Generics


I have two seperate Excel documents that I have in my doc. Because the files both have the same icons, they look the same in the Dock.

So to make it easier to distinguish them, I made custom icons. I copy/past them on, and then re-drag the file to the Dock.

Okay, so this works... for a while... I've had it happen 4 times now, that after about a week or two the icons revert to the generic Excel ones.

I don't turn my computer off much, but it does seem to be related to either new Software Update installs and/or system restarts.

What can I do to get these icons to stick?

Is there an OS X equivelent to the Classic "Desktop Database"?