RHEL 5.3 portmapper service stops running on NIS server

Running an old RHEL 5.3 server (still on it because of legacy software) that also serves up NIS. Starting about a week ago the portmapper service began stopping. When this happens then the yp*, NFS and RPC services continue to run but aren't working. We end up having to restart the portmapper and the above services to get things running again. I've had trouble finding ways to troubleshoot this, very little information on portmapper as it's been replaced by rpcbind. There's also not a lot on portmapper logging that I've found that I could enable and review to help troubleshoot. I know the best solution is to upgrade the server to latest and greatest but that isn't possible for a while. Looking for someone out there that is familiar with portmapper and can offer suggestions to troubleshoot this.