Xbox Live with Powerline/Homeplugs Issue (Belkin)

I would say its the plugs. I've seen Belkin homeplugs have issues with Xbox before, in fact many Belkin products (mostly routers) have some or another issue with Xbox. Does the Xbox work ok wired direct to the router (you should test that)

Well the only thing I can really think of is that the computer is going to be a lot more lenient about disconnects, most computers will attempt to reconnect for up to a minute before they actually tell you there's a problem. The streaming of TV would also be a little different than the way the game works because it's a 1 way transaction with a buffer, so if it loses connection it will just read from the buffer until the connection is established again.

The xbox however isn't going to be able to buffer gaming. If it's losing connection it will have to disconnect because most everything it does is real time, I'd only be able to suggest hooking up another xbox to the connection and seein how that works since that's the only real device that will operate on the same principles. It's not a computer, it uses those components but ultimatly it's whole operating system fits on a flash rom chip on the motherboard, so it can't be as complex and forgiving as any computer.