Phone reboots on its own

[sirtronics]

I am running the latest "safe" version of CM 9 on my Sprint Nexus S4G.
It was running great with no problems but then it all started.
I would be doing someting and the phone would become nonresponcive and just lockup.
after about 20 secound it would just reboot.
It I opened the browser (built in one) it would lock up and then reboot.
Or just reboot for no reason at all.
I firgured than some how the install went corupt, so I did a clean install:
?Wipe Data/Factory Reset
?Wipe Cache Partition
?Mounts and Storage -> Format /system
?Mounts and Storage -> Format /boot
?Advanced -> Wipe Dalvik Cache
and reinstalled everything.
worked fine for a day and now again it locks up and then reboots, not as often.
At the moment use the browser, but I am sure it will ock up again too.
I need help is there something with the phone or is it software.
Someone said my battery is screwed????
help please....