PC’s dropping from network neigborhood?

I have a 2 subnet network and I am at a loss as to why computers at one sight keep disappearing from network neigborhood. They are still there and I can ping them and connect by their computer name in internet explorer.
I also stopped the browser service on the local bdc and get the list back up temporarily, only to have it go away by the end of the day. What am I missing here?