This has happened to me on occasion and most recently in 6.5. The fix I use is to save the message in the Work in Progress folder, then re-open and it's fine!
Another problem a couple of my users (myself included) have experienced is when you're typing a rather lengthy message and hit the "enter" button, GW crashes causing you to lose the message!!! This occurs in 6.0 and 6.5.
BTW, these issues occurred only in GW...reinstalling the client didn't fix the problem.
Regards,
Stars