Vista - Desktop Icons keep changing randomly

I have added a number of document or website link shortcut icons to my
desktop - some are in folders containing a group of shortcuts. I have
changed the folder and shortcut icons from the standard explorer icon to
more meaningful ones stored on my hard drive as ico files.

However - sometimes and for no apparent reason - certain icons randomly
revert to the explorer icon. Right clicking on the desktop and selecting
"Refresh" has no effect. After a reboot - the icons may - or may not -
correct themselves. It is worth noting that it is only the icons I have
allocated that randomly change and it is not always the same ones. Icons
associated with programmes seem to be OK. (Word, Excel, Acronis, Nero etc)

If I right click on the faulty ones the heading of the web document tab
still shows the correct icon but I have to go through the procedure of
changing the icon all over again to get the specific icon back on the
desktop.

I have searched the Microsoft knowledge base in vain.