Well, the first thing to be sure of is that you're running it on fully charged high capacity batteries.
If it's still fading... not something I've seen, I must say. It could be a faulty connection at the power, I imagine (if it's actually dying as opposed to continuing but with no screen - testable with, say, the MP3 player or mplayer). Otherwise I personally have no idea. But I'm not an electronics buff, so I probably wouldn't do!
Tbh, though, I'd probably just be tempted to send it back wherever he got it, and get it fixed. The machine hasn't yet been out for a year, so it's definitely in guarantee time, even if it was bought right at the start...