cut and paste within gedit give spurious results

I am using gedit to edit source code files (fortran). When using cut and paste, the paste often goes to completely spurious and unpredictable positions (sometimes more than one position for a single paste!), when using either the mouse or the cursor to define the insertion point. This happens on at least three different computers all running Ubuntu 10.04. I have been using gedit to do this for more than 2 years and have never had this problem before. It seems to be getting worse and started happening after I upgraded to Ubuntu 10.04. I have searched the internet and can find no explanation of this strange behaviour. Any advice would be gratefully received.