3GS shuts down spontaneously?

Hi all,

Been browsing various threads here and finally decided to make first post! 🙂

I have a 3GS (Model MB715LL with iOS version 6.0.1, 10A523). It's been faithfully connecting me to the world since 2009.

Lately, I've been having issues of spontaneous shut-down, where the phone will just turn off on its own for no apparent reason.

This has happened during activities such as sending group iMessages, browsing routes using the Google Maps app, and even just composing e-mail messages using Mail.

Initially, I thought battery lifespan might be the issue. Starting the day at 100%, battery level would fall to 60-70% by mid-day while being on standby.

Under Settings > Usage, 10.8 GB is available. Highest memory consumption is Spotify (109 MB), down to Google Voice (4.2 MB). I don't really have many additional apps on this phone: Spotify, Pandora, Songza, Whatsapp, Talkatone, Google Maps, Google Voice, and NO music, video, or photos.

iCloud and Location Services (except Maps app) are both completely off.

Mail, Contacts, Calendars is set on Fetch New Data hourly, so Push is off.

Brightness of the screen has the slider set somewhere between 25-30% of the way.

Obviously, with a 3GS, I have no access to Siri.

EDIT: I've also tried a couple things using the iTunes software as well as on 3GS itself (Settings > Reset), including Reset All Settings, Reset Network Settings, Restore Factory Settings. I've also tried just about all of the suggestions from the official Apple Battery page (http://www.apple.com/batteries/iphone.html), but none of it has helped significantly.

The mind-boggling thing to me is that when I compare usage to my partner's phone, we start out the day with 100% battery level. By late afternoon, my phone is < 50% battery remaining, while my partner's phone, which has more apps installed, push notification, etc. is consistently > 90%.

Any brilliant insights would be much appreciated! Thanks for reading!