I finally tracked down how query for a non-existent map brought down my NIS server. ypserv never frees the memory allocated for non-existent maps. Here is a patch which seems to help a lot. I don't know if there are any other memory leaks in ypserv.
Created attachment 77606 [details] A patch to fix one memory leak
I don't think this patch is needed for ypserv-2.5 (which is in 8.0 final). It verifies that the _db_open call didn't fail before strdup:ing domain and map. Can you verify if it works for you?
It seems ok. It will be nice to have an update for 7.x.
Please see http://rhn.redhat.com/errata/RHSA-2002-223.html