The best way is to post the crash log next time it crashes. That will contain specific information that will help find the problem. Logs are cryptic but there are people here who can make some sense of them.
I'm running 5.1.7 under 10.6.8 on both my iMac and MacBook Pro and it is completely stable. However I have no extensions installed. Incompatible extensions are a common source of Safari instability