My phone does the same thing now. My phone never did this when I first got it a couple years ago. I use GPS a lot because son plays on a travel hockey team. I could drive 3 - 4 hours away with the phone on and plugged in using the GPS without any issues. Now, my phone gets so hot in about an hour of the same type of use and it shuts down.
I thought that maybe there was something wrong with the phone because of age but now I see it could be something different.
• Service: T-Mobile
• Device: Samsung Galaxy SIII [T999]
• ROM: Stock 4.3
• Kernel: Stock
• Modem: Stock
• Launcher: Holo HD
• Recovery: Stock
• Device: Samsung Vibrant 1 [Retired] • Device: Samsung Vibrant 2 [Sold]