Welcome to Discussions, Douglasb37.
The only remaining user-correctable common problem I know that you do not mention is the possibility of third-party (non-Apple) conflicts. If you have eliminated possible third-party software conflicts, I would suspect an intermittent hardware issue.
However, if you are using one of the old external Firewire-connected iSights, the conflict may also be caused by other Firewire hardware. In this case, uninstall and disconnect all other Firewire devices, restart your Mac, and test again to see if your external iSight works when no other FW devices are connected. If so, your problem may be hardware conflict. Click this link for more info about external iSight hardware conflicts.
Any intermittent problem can be very difficult to diagnose, but your Apple Authorized Service Technician (AASP) has all the special tools, test equipment, and technical data necessary to find the problem as quickly as humanly possible. Because intermittent problems are not reliably repeatable, they can be difficult to find, so make a backup and expect to leave your Mac with the AASP.
Without all the specialty repair items available to your AASP, troubleshooting the issue further on your own may take a lot of time. However, if you want to continue to try to fix this yourself, I offer my best suggestions below.
(1) Unless you already have one, make a current backup.
(2) You can reinstall all your Mac OS and all other Apple software after a "secure erase" of your startup disk.
If your problem is software, securely erasing your hard disk and properly reinstalling your software may help. If you need help with this, here is how I would do what I am recommending.
• After the Lion install is complete, download and apply the latest Lion Combo Update, currently http://support.apple.com/kb/DL1484
• Next, use your Mac's (not Lion Recovery's) Disk Utility to repair permissions immediately following the system restart that completes the Combo update.
• Now use your Mac's > Software Update... menu command to check for an install any other necessary Apple software updates.
• Use your Mac's (not Lion Recovery's) Disk Utility to repair permissions again if you installed any additional Apple Updates.
• Do not reinstall any third party items at this time. Third party items can be either hardware (mouse, keyboard, A/V interface, etc.,) or software (including Microsoft Office or Exchange, plug-ins such as Perian, Flip4Mac, etc.)
• Test whether iSight works consistently with the http://support.apple.com/kb/HT2090 suggestions. If the problem still exists now, or if it returns in any of the further steps I suggest below, yours is almost certainly an intermittent hardware problem that requires AASP service.
• If iSight now works properly with the HT2090 Troubleshooting tests, you can add necessary third-party hardware. Add only one item at a time. Test each addition thoroughly and check whether iSight still works before adding the next item. Thorough testing between additions will help you determine if one or more of your third party items is causing a conflict with your iSight.
• If your iSight is still working properly, you can now reinstall additional, necessary software applications. Again, add only one item at a time.
Reinstall additional software from the original disks or download files. Do NOT copy the software from any of your backups because the backup copy may contain the damaged item that is causing your iSight trouble.
Start with the Apple apps that are not a part of OS X (such as iLife.) These Apple apps should not cause any problem, but it is wise to verify that iSight works with each one after you reinstall those Apple apps but before you begin adding any other necessary third-party software apps (if any.)
• Check every addition to be sure they are not installing "login items" that may be launching software when you restart your Mac. You can add the startup item back if desired AFTER you are certain that an item is NOT part of your iSight problem.
• Do not copy your data files back until you have tested all the individual additions of apps and third-party items.
(3) If nothing suggested here resolves your problem, your Apple-Authorized Service Provider will offer the only solution I can imagine being successful. There are no user serviceable hardware parts of iSight.