Hi Gustavo!

>
> So... the same database (the same file!) gives error five times with gbak and gfix, and a few minutes later, without doing nothing, the file is magically good!
> Am I crazy? Perhaps this is true also ;-) but... does anyone have an idea of what may be happening?
>

This sounds like some AntiVirus program / Windows SystemRestore / imaging program / backup program messes with the database file.