Since the gist of this thread has clearly laid out that by this time all possibility of the issue being hardware related has been laid to rest, it may be that somewhere along the way the video driver may have become corrupted, which is different from the driver being programmed incorrectly. As such, performing a complete backup of all your files in Time Machine (onto an external hard drive), then TOTALLY wiping your internal hard drive clean, and then doing a clean re-install from your original restore CD's may solve the glitch. After the re-install is completed, create a generic 'admin' account, re-associate your computer with your Apple ID, then re-install Lion. After that, run 'software update', restart your computer if it asks, then use some of the things that used to crash your system, such as screen savers, youtube videos, and whatnot. If the crashes do not reoccur, the problem may have been licked, but before deciding all is well, run Migration Assistant and choose to restore from Time Machine, but only allow it to restore your user account(s) and 'other files', but not applications or settings. After the migration, log in to your restored account(s) and see if the freeze happens again. If it doesn't, then problem solved. If it does, then restart the computer and log in to the generic 'admin' account and re-test. If it only happens while logged in to your restored account, then the issue may be related to some kind of corruption in the user account which followed the backup and restore process, which may be hard to track down. At that point, the quickest solution would be to migrate your user data to a fresh account that does not include anything from your user 'Library' folder (hidden in Lion). Now, if you choose to run migration assistant again to restore your applications, and the issue happens, the issue may have followed a corrupt application. So, I always recommend that you should in fact just re-install your applications from scratch and skip the headache of possibly having to start over, because, after restoring them from backup it is very difficult to weed out issues that may have piggybacked the process. A freshly installed copy always has the least risk of having to start my entire above process from square one all over again. Just remember - Backup FIRST! I'm not responsible for any lost data if that step is skipped!
If the above sounded too confusing, then I suggest bringing the computer in to an Apple Store or AASP and asking for the apple-certified technician to do this for you, since I do not like to see people lose their data due to miscommunication.
