Strange, I'll try as soon as possible.I've been watching CPU for Safari 3,.0, and the highest I've seen on my machine, with 6 tabs always open, is 22%.
Maybe completely trashing Safari, it's .plist and move the Safari folder to your Desktop from User/Library, then reinstall.
The activity window (from the Window menu) might provide the info you need.
You can also activate the debug menu, which could be helpful with some page-loading situations, I suppose. I know there's a way to do this from the terminal, but I use Onyx, which has a GUI to enable Safari's debug menu, along with other system settings that can be interesting.
defaults write com.apple.Safari IncludeDebugMenu 1
Now relaunch Safari and check the "Log JavaScript Exceptions" menu item in the Debug menu. In Safari 1.3 and above, select the "Show JavaScript Console" menu item and the JavaScript Console window will open to display JavaScript exceptions. For Safari versions before 1.3, JavaScript exceptions will appear in the Console application (/Applications/Utilities/Console).
Safari 1.3 and above supports explicit logging of arbitrary information - similar to Objective-C NSLog() - function by using window.console.log() in your JavaScript. All messages are routed to the JavaScript Console window and show up nicely in a dark green, to easily differentiate themselves from JavaScript exceptions.
I've tried as you said and I've found that the problem is with my bookmarks.3mors - if Safari works after that, take your bookmarks.plist file out of the Safari folder on your Desktop and put in the Safari folder in user/library/Safari
The only time safari 3.0 quits on me is with a site that has flash, but not all the time.