Vista - Thumbnail images when viewing Extra Large Icons with Explorer

With Vista Explorer, when using the view "extra large icons", the folders
can display a thumbnail of a picture inside the folder. I use this method to
browse music albums, being able to see album covers.

When initially selecting this view, Explorer takes some moments to create
the thumbnails for each folder. On subsequent viewings, it is much fasterm
suggesting Explorer has catalogued these thumbnails and does not need to
generate them again. This would make sense.

I have noticed however that sometimes Explorer acts as if it needs to
re-generate these thumbnails, and takes as long to display them as the
initial time. This is despite having gone through this process a number of
times before. This is very annoying and time-consuming.

Any ideas why this is & how to rectify the problem ?