Angel,
Does this happen with ANY project, or just this one? If you make a brand new project with just a few simple objects do you still have the problem?
If you haven’t done so already, does it still occur after you have rebooted your computer?
I see that you are using R13, which I also use (Mac version, however). If you’re still having the problem, post your project and I will test it for you.
Good luck.