Typing problem occurring as soon as I delete something in a composing email

This problem has been bugging me for sometime now and I'm sick to death of it.

I'm running Mac Mail 8.1 and the problem only seems to appear when I compose an email in Mac Mail and no other application.

I'm using a USB keyboard.

When I write an email at first glance things seem ok, until I need to delete something. Then all of a sudden the cursor seems to have a mind of its own and deletes wrong characters and the chances of composing the email from then on is futile.

My work around is to write the entire email in any other application and then copy and paste it across. This is time consuming and shouldn't be needed.

Quitting Mail and relaunching it doesn't help. Restarting the Mac does help for the initial few emails, but sure enough the gremlin is back and composing emails is a nightmare.

Please can someone help?

An example of a written email would go something like this: All I want to do be able to etesomgggething ddwithout it doing this!! Grrr!!