Since the Motorola DCH6416 is a CableCard specific device, I have a sinking feeling your cable/TV service provider remotely disabled the hardware. Remember many CableCard ready devices do perform 2 way communications with the TV service provider; specially upon start-up/booting when they sync up with the provider's network/service.
You said it yourself, you can get the device to indicate its starting to boot, but returns an 8888 code & then resets itself. This sounds like a device that was remotely disabled by the cable/TV service provider to me...
Cable TV service providers around me, routinely remotely scan for & disable/block 3rd party hardware, which was not registered through them. They do this to prevent theft of cable TV/internet services & ensure only compliant/legal hardware is used on their networks.
I also know that some cable/TV service providers also disable 3rd party cablecard devices, when the user have not purchased one of their cable cards from them (i.e. when users are trying to use a hacked cablecard or smart card to bypass their security measures).
I ran into a similar issue with an old Motorola cable modem I used to own. I wanted to use with my cable TV service's ISP, since I kept having problems with their provided cable modem. I was unable to access or use my own hardware on their ISP network, until the hardware was registered to my account within their ISP's network & they released the hardware lockout they set to prevent its usage.
However I do agree with Miller, you should contact Motorola & see what they have to say. If Motorola cannot tell you what that 8888 code means or of they cannot unlock it (if its securoty locked), then you will likely need to contact your cable/TV service provider, and ensure you do not need to register the hardware with them first &/or unlock the hardware.
Of course, this is just my 2 cents...