Just an update:
Once it locks up from the browser, it locks out all browsers on different computers - the local file is still fine and I can interact with it. The only way to reset is to quit Filemaker Pro altogether - sometimes needs a restart.
I can usually interact with it between 3-10 minutes then it locks up - I have the timeout set to 59 minutes.
After it locks up and when I go to quit Filemaker it thinks people are still signed in and because it is no longer communicating with Browser, it won't quit. I have to force quit.
After spending all this time developing this database, this seems like a show stopper. Any suggestions?