You need to let it keep cycling until it finally loads up. I have been having a similar issue with my phone lately. It always took a long time to reboot (like 15 minutes) but in the last couple of weeks it has taken up to an hour. Mine does the exact same thing yours does: it boots, displays the bar at the top with the signal, battery, etc, then vibrates and reboots...over and over. Every so often it will run the media scanner as well, then cycle again. You have to leave it and let it reboot itself. I recently added and updated a bunch of apps so I think that has something to do with it. From what I've read online the way Samsung set up the memory management (software-side) is pretty crappy and this is probably the cause.
I use my phone for work and having it out of commission for an hour for a simple reboot makes my life very difficult. All that because the phone froze on me to begin with and I was forced to pull the battery.
Hope that helps.
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