Whenever I press the home button twice the iPhone restarts
I have an iPhone 4S with ios 5.1
Whenever I press the home button twice to close some app or something, the phone restarts.
There was this same problem yesterday and then automatically it got normal.
Before that ,once with this issue I did a recallibration ( opening an app and then pressing and holding the lock button until the switch off slide appears and then holding the home button until I am back on the home screen) which solved my problem.
*but it solved my issue just once.
This has happened many times with this phone , even when it was running on iOS 5.0.1.
Now I'm sick and tired of it.
Please help me finding out a solution.
Thanks in advance.
iPhone 4S, iOS 5.0.1