I'm experiencing the same issue. I often find the ROKU has rebooted overnight. It's extremely frustrating. It also happens sometimes while I'm scrolling through menus. My Roku Ultra (4660, 4661, 4662 series) has been doing this since the last firmware update.
Haven't been able to find a solution. As for the router being a possible issue, I disagree. I have been using the same Mikrotik router for a long time and only recently started having this problem on ROKU.
You can sometimes get the previous version of the firmware during the rollout of a new version from the "Secret Screen" (press Home 5 times, FF 3 times, Rewind twice). Choose "Update software" and it may offer the previous version as an update.
---A problem clearly stated is a problem half solved---.
Don’t know about freezing but there was a man’s face on the rock in my aquarium screensaver. It was pretty non descriptive and disappeared. Now says Roku. No idea why. My son saw it also or I’d doubt it. Any ideas?
Try removing the channel from the Roku home screen by navigating to the channel tile, pressing the * key on your remote and choosing 'Remove channel'. Then restart your device from Settings>System>System restart. Once your player starts up again, add the channel back once more.
Does this clear up the issue you are seeing? Please keep us posted.