My iPhone 4s keeps restarting itself. Does anyone else having the same problem?

Try doing a hard reset on the device... Press and hold the Power button and then press and hold the Home button until the Apple logo shows, then release. See if it helps.

If not, try to backup the device and erase and setup as new.

http://support.apple.com/kb/HT4137

i have the same problem, don that, didn`t worked! it`s a hardware problem!