@TheLex I'm not certain how you're auto-starting a specific channel, but if you say you've done it, great.
As to Roku "powering down", you didn't specify what model Roku you have. Some of the latest models have a setting called Power Saver, which doesn't actually power the system off, but does put it into a deep sleep mode. If you're seeing a true system reboot, it sounds like it is losing it's power. How are you powering your Roku? If it's connected to a USB port on a TV, then the power is likely being shut off after some internal timer on the TV. If you plug your Roku directly into wall power that should stop happening.
There's also a setting under the Network menu labeled Bandwidth Saver. This is supposed to stop streaming if the remote isn't used within a four hour period. Since I never go that long without touching my remote, I don't know if it simply stops the stream or closes whatever channel is open. If the latter, that might be what's happening with yours. Try turning the saver function off to see if that resolves the issue.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee, just another user.