Ask for specifics in the PPro forum. In so many words: What do I know about multicam? I suspect installing CS6 has simply updated the CoDecs and one of your footages now behaves differently. If so, uninstalling CS6 and possibly running a repair /re-) install of CS5 should fix that.
I can't seem to find how to uninstall CS6.
Reinstalling CS5 keeps giving me fatal errors and warnings and stops the process.
I don't see uninstall on the CS disc or in my programs folders?
Found it. Uninstalled it. Reinstalled old CS5 from scratch. Same issues.
How can I restore codecs?
Moving this discussion to the Premiere Pro CS5, CS5.5, & CS6 forum. Did you receive any error messages when reinstalling CS5? Also which operating system are you using?
I found the full application cleaner thing and was able to reinstall all.