[1.4.9.1] Missing Sub Categories Issue

Hi,

OK, this is a strange one, I can't identify why it happens or any pattern to it but....

Sometimes when I go into the back-end and open categories, the subcategories are missing!!, if I select a category, do nothing but save it... when I come back to the category list... lol and behold all the subcategories are back.

It doesn't affect the front end, but it's getting to be a real pain having to check it before I mod a product, just in case the links aren't there when I save the changes.

Any advice gratefully received.