General :: Galaxy S4 Running Android 4.3 Keeps Turning Off?

I have a Galaxy S4 running android 4.3. Recently it's been randomly shutting off. I ride a motorcycle and most of the time when I get home the phone is off. If I try to turn it on it just goes into a reboot loop and the only way to stop it is to plug it into the charger. At first I thought maybe that the power button gets pressed somehow in my bike jacket pocket and hard shuts it down. But it keeps doing it.

Also I was using it whilst sitting in my house and it just randomly shut off. I'm not too sure whether it does it at a certain percentage or not or whether it only does it when a lot of memory is being used, which I doubt as I'm obviously not using it when it's in my pocket.

Another thing, and I don't know whether it's related or not, but when it shut off the other day whilst I was using it I thought If I took the battery out and back in would it let me boot it up without the need for a charger. But no it didn't work. I plugged it in and then when it came on the language had changed to Slovakian.

Looking on other posts about this some say it's a bad battery. I have AVG anitvirus so I doubt it's a virus although maybe one that isn't yet recognized by AVG? I would like to try and sort out the issue without factory resetting my phone. I have loads of apps and would take ages to back it all up and also the phone is rooted and that's difficult to do as well as install all my root only apps.