iPhone 6 restarting on it's own

My iPhone 6 has been restarting constantly on it's own. It started doing this after updating to iOS 8.3 recently. I contacted Apple support. They ran diagnostics on it and told me there were no issues and that I should restore my iPhone in factory settings. Now, after doing so, I'm still having the constant restarting issue, and it doesn't stay on long enough to restore the backup. Does anyone know how to resolve this issue?