I don't know if you went through the troubleshooting that I mentioned earlier.
http://www.mac-forums.com/forums/os-x-operating-system/93819-basic-os-x-mac-troubleshooting.html
If you have, then under the circumstances, as this could go on forever trying to resolve this, I would suggest the clean install.
The reason being that if the system works properly after a clean install, then you know that it is something corrupt on your system as it is, and then you can restore your system with the last working backup that you had before things went all pear shaped.
CLEAN INSTALL
Back up onto an external hard drive.
Then, insert your SL disc, and restart holding the C key when you hear the tone.
After you choose the language and press continue, go to utilities/disk utility in the menu bar.
Choose the HD (second in the left column) and select erase disk which by default will and should be formatted as Macintosh OS extended (journalled).
Once erased, exit disk utilities and continue installation.
(Once installation is compete, it will ask you if you want to restore from a time machine backup or using migration assistant later)
SELECT LATER as before you put anything back, you want to see that the system is now functioning properly!
If it is, then there are various methods to restore.
You can manually move applications back which can be tricky, reinstall fresh applications that you know are fully compatible, or using time machine or migration assistant.
The photos, documents, movies, and music are easy to move back manually.
If you want to restore from the last working backup, I would choose time machine, as migration assistant won't allow you to choose which time machine backup you would like to restore from.
Restore from Time Machine
Boot from SL DVD (Holding C at startup)
Choose language and click continue
Go to utilities in the menu bar and scroll down to restore from time machine backup.
This will guide you through the selection of the HD you wish to install onto and the backup you wish to restore from.
Once your done you should be all good.
Good luck, and fingers crossed for you.