Hi Friends,
I also experienced the same problem,I too thought that my Wifi is making this problem
but my Wifi is not a cause for the problem, Also I rooted my phone and no problem from that
also. Then I checked my battery's power with my tongue, I didn't feel the power in my battery.
Then I used a thin pin like instrument to wipe out some minute dust on the battery's output.
Now there is no issue with my Phone, It's not shutting down. Even you feel the power on your
battery, clean that once. Consider this before buying new Battery.
I hope this will fix...
Regards, Ram