Safari will not "keep in dock"

I first asked this on the Safari forum, but I should probably have put the question here:

Recently Safari (v3.1.2) has refused to stay in the dock. If I drag the icon to the dock from the applications folder, it disappears in a few seconds. When the application is open, it's icon sits on the right hand side of the dock, just like any other application which is not normally in the dock. Whilst it's there, I've tried selecting "keep in dock" from the right-click menu. But when I quit, it disappears again. Also, whilst Safari is open, if I try to move the icon from the far right to any other place on the dock it zips back to the right hand side after a few seconds, and then disappears when I quit.

I have searched this and other forums and it seems like I'm not the only person to encounter this problem. The only "solution" I have found posted is a complete re-install! I have tried deleting the dock.plist and dock.db files. After I restart, the default collection of dock items appears, including Safari. Seconds later, however, it disappears. I just cannot get it to stay where I want it and it's driving me mad! Any help would be much appreciated. Thank you!