The most unstable clustered database I have ever come across was suffering from broken TCP drivers. Never assume a cause until you have actually tracked it down.
I agree with itaborai83 that individual node crashes shouldn't be as big deal with the consistency model and redundancy offered by Riak. That is one reason you might go with Riak over something that offers stronger consistency, but is more picky about node crashes and recovery.
I think that the cluster is supposed to be stable. The nodes, not so much.