There are a number of alternatives mentioned in this google search for terms you mentioned above.
A few of the options mentioned are: full discs, permissions issues, "hung" or "stuck" svnserve processes, and hung transactions... You may need to try a number of these different issues to see what exactly the problem is in your case.
It sounds like you are using the bsddb SVN back-end? You may want to try migrating your repository over to the fsfs back-end (the default for several years now). It has, in my experience and from that of a number of others I know, proven more reliable than bsddb. Though you'll want to research how it interacts with NFS -- I haven't used it on NFS.
Another option you may want to consider is moving away from using SVN over NFS and instead running it over SSH on a server with the SVN repository locally stored. This is how we do all our SVN work, with fsfs backend, and I can't remember the last time we had SVN repository issues.