iPhone crashing 1 minute after I turn it on, battery fine

Hi, my iPhone has been working perfectly for a year or two now and just this morning it crashed a couple of minutes after I turned it on. It always instantly starts rebooting, but no matter what I do - keeping it locked, closing all open apps, turning off wifi, etc - it crashes after a minute without fail. I tried plugging it into my computer and doing a backup but it just crashed a minute in like it usually does. The battery is fine, it's at 100% at the moment and lasts a full day before needing a charge. I'm on a 3GS running iOS 5.0, any help appreciated :) thanks.