Offline mode isn't used/tested much so you may want to see if you crash while actually playing on a UO shard.
On the installation manual page (
http://www.iris2.de/index.php/InstallationManual), it says to patch up to 6.0.9.2. This means you may want to patch up the client to see if that alleviates some of the problems.
If you'd like Iris to crash less, please post a description of what you were doing when the client crashed, and along with a copy of the stacktrace.log in the bug forums. This allows someone to work on solving the problem.