This problem started immediately after updating to iOS 11 and has continued through all updates.
iPhone 6S - Presently running iOS 11.0.3
There are no third party parts in this iPhone. It was serviced once for a warranty battery replacement by an Apple Authorized Service Provider.
The Problem - Crashing/Freezing
Various apps (seems any app can be affected including Apple apps like Safari) are freezing up, usually on launch. Sometimes they crash and close down within seconds. Other times they freeze up for anywhere from 30 seconds to 2 minutes. The phone is completely unresponsive during this time (no app switcher or sleep/wake), with the only exception being I can hold home & sleep/wake to reset. When things resume, any of the previous button presses will be processed, however. Sometimes it happens while an app is running, and sometimes the app or portions of the app become unresponsive while app switching works normally.
Regardless of the app, once this has happened once, audio playback starts to break down. The system starts to confuse iTunes and Podcasts. Control Centre or Apple Watch might not show the correct playback status. For example, while listening to a Podcast there might be a "Play" button. Pressing it will stop playback and play iTunes music. Occasionally (once), it started working again for a moment, then stopped again. Often Apple watch can't control playback volume at all. Removing AirPods from my ears and reinserting may cause a Podcast to stop playing and Music to start, or it may not pause playback at all.
The only way to fix this is to turn the phone off (hold sleep/wake and shut down) or hold home & sleep/wake. But within hours the problem returns. I am restarting my phone 5 times a day right now!
I tried "Reset All Settings". No change.
In my searches for solutions I heard about a similar problem but it seems it was associated with a third party display and was fixed in the current update? I have the same or similar problem but no third party display and the latest iOS update is installed. All of the apps I use are up to date also.
What else can I try?
I tried posting this question on discussions.apple.com but I keep getting the unhelpful error message "You are not allowed to create or update this content". Infuriating!