sounds like you are having connection issues. if you can't even watch a youtube video in the stock youtube app, you have connection issues.
can you provide a link that you are trying to watch in the youtube app? lets see if I or others can play the link on our own units.
what carrier are you on?
and let's, for now, uninstall the flash player apk so that the phone can be as stock as possible.
you are NOT rooted and you have fastboot deactivated yes?
also, does this happen on wifi AS WELL AS cellular?