Is your Roku device powered by plugging it into a USB port on your television? The ports on many televisions only supply 0.5 amp, which is not enough to power a Roku reliably.
It is recommended that you power your Roku device via the power adapter that came with it, plugged into your house power. If you no longer have the adapter that came with your device, you may have another one handy that can be used -- just make sure the tiny print on it confirms it has a 5 volt dc output of at least 1 amp.
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