[CentOS] CentOS automount 99% CPU utilization -- NFSv3 vs NFSv4?

I have several CentOS 5.2 workstations in a Sun Solaris NIS domain and
Solaris NFS servers (all Solaris 8).  Frequently the auto mounter hangs with
very high utilization -- I think it might be due to the Solaris 8 servers
only using NFSv3 and the CentOS wanting to use NFSv4.  How can I set the
system to use NFSv3 globally, even for auto mount maps from my ancient
Solaris NIS domain (which I wold like to upgrade, but that's another
story...)
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