Well, I found this on a link about a very similar problem:


The ORA-600:[729] error is likely caused by a bug which effects 8.1.5 where a shared server process can encounter an ORA-600:[729] error when a process using database links disconnects. The bug is fixed in 8.1.6 and included in the 8.1.5.1 patchset release. The 8.1.5.1.1 patchset release for NT is available for download from the Patches option on MetaLink Home.

I've also noticed a couple of other links relating to SP5.

The common suggestion though is to set:

sqlnet.expire_time=0

in the sqlnet.ora file on the server, and see if that helps. The other suggestion is to raise a TAR with Oracle - I think they will come up with quite a few hits relating to this. Although you will need the whole ORA-600 message and any associated trace files.