170 F is way to hot..
I agree with above replies, it sounds like an app issue since you stay it started a few months ago. But if the phone has been getting that hot why wait a few months? Its normal for it to get warm after a restart because your opening all the apps back up and syncing is starting from scratch for all the background apps. But again, shouldnt get "that" hot.
Unless you also have a case on it.
The other thing is how accurate is the app your using? CPU temp can get super hot but i tend to go with battery temp as that is more of a critical temperature. The CPUs are designed to get really hot then throttle back when it gets dangerously hot.
But the battery is what can get damaged by heat.
I dont see how you could hold the phone if it was 170F (phone not CPU).
IF the phone is to hot to touch / hold then theres a problem, if its warm your fine. Also if its only when booting up, eh maybe its fine.