My iPad2 starting freezing

This started happening around Thursday. The touch screen do not response anymore. Hardware button works. Rotational sensor works. A hard reset (home+power for 10 seconds) sometimes work. Being a tech person, I systematically deleted apps that I installed to that date, but that was not the problem.

I wonder if it is related to network -- i.e. if it is access the wireless network (checking mail, push notification etc) and gets locked up.

Any one else starting to having this problem?