I have the S2 for 3 days now and just now and yesterday the phone just shut itself down and restart again. I think it happened 3 times in two days. So, I want ask if anyone experiencing the same thing and should I return the phone and get a new one?

It is most likely caused by an application that is on the SD card. Not all applications are 100% compatible with being on the SD Card.

• Move all applicatioins of the SD Card on to the main phone memory
• if the above does not work uninstall applications until the problem is resolved.

Even if it is not an application being on the SD Card that is the issue it is most likely an application.