my iphone restart in every 15 minutes when i left it on standby....what can be the problem? my iphone is 3gs with iOS 5.1.1

Basic troubleshooting steps outlined in the manual are restart, reset, restore from backup, restore as new. Try each of these in order until the problem is resolved. If you've been through all these steps and the trouble persists, you'll need to bring your phone to Apple for evaluation. Be sure to make an appointment first at the Genius Bar so you won't have a long wait.

Best

GDG