Thank you for your post, and I apologize for the late reply.
It is difficult (and frustrating) to identify the cause of issues that can't be replicated on demand. Since you are able to select your host at times, more detail about the computer configurations would be helpful.
1. What do all the Macs with the issue have in common? Login Items? Third-party applications? Other networked software?
2. Does each client launch FileMaker Pro in the same way? Are there any opening or startup scripts being used?