My iPhone 4S keeps freezing and restarting?

I have an iPhone 4S which I bought in December 2014. It's running iOS 8.4.1. Recently it started to freeze when I'm trying to enter the passcode. It would be unresponsive then restart itself. Occasionally I can get into the phone, but it would also freeze and restart when I try to do something. Then it logged my iCloud, mail, and notes account out. During this time it was running smoother (slightly). Then I logged my accounts back in and the freezing and restarts started happening again. It's really annoying because I can't get a new phone til December when my tab is paid. The phone is almost unusable. My iMessages won't send and my phone vibrates repeatedly.