This forum post at AnandTech cleared up an issue I was having with my Roku 2 XS. Once a night, my Roku would stutter. Then a few minutes after the stutter it would freeze and reboot. Normally I was just watching Netflix, sometimes I was watching the Plex app. From what I can tell, the chip was overheating. There is no ventilation for the Roku and while that keeps out dust it means that it can overheat too.
The forum post details taking off the bottom cover of the Roku and applying a heat-sink to the chip. It is very easy to do. Here is the heat-sink I bought from FrozenCPU.com and recommended from the forum post. Shipping will probably cost more than the heat-sink. It comes with a sticky thermal pad, so installation is just peel and stick. The heat-sink will sit off-center from the CPU to avoid other parts of the Roku, but it still covered the CPU completely and worked like a charm. I’ve had no unwanted reboots for several weeks since I applied the heat-sink.
2014-09-02 Update: After many weeks of working normally the reboots are back. Not every night like before, but once or twice a week. I’m going to remove the cover to let some air flow through and try to figure out if this really is a heat issue.