This really sounds like a database issue, server or permissions issue. Inconsistent issues like this are extremely difficult to troubleshoot at best as you can imagine. There could be a conflict even, perhaps a certain browser configuration is having a problem. If it was happening all the time, the issue might be traceable, something like this could be a 100 different things, not even related to VFBPro causing the issue.
You may consider using our pro version, we certainly have no reports of this issue in our pro version.
Thanks,
Joseph
VFBPro Support