System Preferences won't (often) open

Question

From the Dock, from the Apple menu, from Applications - most of the time System Prefs bounces in the Dock until I force quit.
I trash the plist file and then it will open, sometimes from the Dock, sometimes from the Apple menu, sometimes from Applications - not reliably from any of these though. Quite often I have to trash the plist file again and try again.
I've tried making S.Prefs a login item and, on most startups and restarts, it will then open, but not always.
I've created a new user account and there's no S.Prefs problem when I log into that.
Once before, I installed the combi update and that cured things for a few days.
Today I trashed the plist file, emptied the trash, repaired permissions (nothing there about SPs), downloaded and installed combi update 10.6.3 (was already on that via SU when it became available) - and the problem's still there.
Would be very grateful for suggestions as to what to try next.

Answer 1

Startup in Safe Mode (It will take more time to startup in Safe Mode because it runs a directory check.)
If your System Preferences functions correctly that way, go to System Preferences >> Accounts >> Login Items, and remove them. Boot normally and test. If not go to Users/(yourname)/Library/Contextual Menu Items and move whatever is there to the desktop. Then do the same with /Library/Contextual Menu Items. Lastly, try moving ~(yourHome)/Library/Fonts to your desktop and restarting.
Log out/in or restart, if that sorts it start putting items back one at a time until you find the culprit.
-mj