Hyperlinks disappear randomly
 

I am using Excel 2003. I have hyperlinks in a column that appear to be there
one moment and then disappear another. I have reset the hyperlinks, gotten
out of the file, reopened the file and some hyperlinks disappear while others
remain. Also, after reinserting the hyperlinks and working somewhere else in
the document, I have noticed that some the hyperlinks have disappeared again.
These hyperlinks have originally been copied from a webpage and all begin
with http://www.... Why are they disappearing on me, and how do I keep them
permanently?
Thanks.