General :: Galaxy S4 Randomly Turns Itself Off And Goes Into Bootloop

I got the phone from 3 UK on a contract in July, so phone's pretty new and not rooted or anything.

Yesterday the phone started turning itself off at random times quite a lot.

Battery could have been over 60%, at 30%, it was never with the same charge that it would happen, also never when doing the same thing on the phone.

Usually when it's about to turn itself off, I lose signal completely, then the screen just goes black.

When I try to turn it back on, it usually dies again at the 3 logo or even at the Samsung one and I have to try and turn it back on again. Sometimes I had to pull the battery out and connect it to the charger to make it boot up ok.

After a couple of times it did that, it started going into a bootloop when attempting to turn it back on again after the random shutdown. Phone wasn't hot either.

It had happened a few days before but just once not this many times in one single day.

I removed the microSD card, since it makes the phone think the device has been modified and won't let me update, but the microSD works fine in my old Samsung smartphone and even with that removed, phone still turned itself off and then went into a bootloop.

I think it might be the battery. Last time I was using the phone unplugged yesterday night, the battery went down to 33% charge in less than an hour and phone turned itself off. After managing to make it boot up again and connecting it to the charger, it was saying 60% charge. I doubt it could have gone up to 60% being plugged in just for a minute or so.

So, I can certainly take it back to the store and have them fix it or buy a new battery and see if it solves the problem.

Is it likely to be the battery that's malfunctioning?

Lots of people on the Internet seem to have this problem and a few mention a swollen battery. They replaced the battery and phone was fine after. Mine seems ok, but I can't be sure.

I don't understand why I seem to lose signal completely just before it switches itself off... it doesn't do it all the times, but a lot of the times yes.
I have other phones on 3 and they didn't lose signal completely and anyway in my house coverage has always been excellent.

I forgot to mention that once, while trying to restart the phone, I got this message, more or less: "Installing custom OS can damage your system or applications. Choose volume up to continue...". My phone's not rooted and I haven't installed any custom ROM!