I have a reoccurring problem with Thunderbird. When typing in a reply the cursor doesn't keep up, it starts falling behind.

As you can see the cursor is over the "L" not beyond it. This makes correcting mistakes next to impossible.
I seem to have had this problem back when I used the early Mozilla mail app. back in the days of OS 9. I am now using OSX 10.4.4 on a Mac G4 450 dual 640 RAM. I have TBird 1.5
Any idea of a solution? I have searched the web without any clue. I know some people have had trouble with disappearing cursors or "magic cursors" that move on their own. This is different.
Any help would be appreciated.