Why do my iMessages stop sending/receiving randomly?

I have an iPhone 4s on Telus Mobility running iOS 8.1.3. It works fine (other than the often occurring freezes and terrible battery life), but randomly my phone will not receive any iMessages (I haven't tested with regular SMS messages because all the contacts I usually talk to have iPhones), and if I try to send out an iMessage to a contact it just sits there endlessly sending the message. This occurs on both Wi-Fi networks and on Telus' cellular network, and to fix the issue I have to restart the phone. If I perform a hard reset of the phone then it goes back to normal, but I don't know why it randomly stops sending/receiving messages.

Does anybody else have this issue? Any ideas on how to fix it? Thanks!