I have two Roku's and if there's not a problem with one its the other. I'm assuming that my device recently updated as that seems to be when problems happen. The device is stuck in a constant reboot loop right after the dancing Roku logo finishes. Why does this continue to happen to these devices? If its not that its the comcast beta that forces the device to constantly reboot.
I've had the same issues with my identical model as yours. If you have an SD card in it, remove it and see if that solves the issue. Use a name brand, not a cheap Chinese knock-off, in the box. I have found that Micro SD card brand works best. You don't need any bigger than an 8gb card. Any more than that is overkill. Hope this helps.