Yeah, that should not be happening, on the slim because the wireless network controller is built in. I wonder if xbox uses the same type of adapters as they do in laptops. If they do, your wireless controller may have come loose. It could also be a software fault, like a driver is going wonky on you. Try to reinstall the most recent system update like unicron suggested, it just might work.
If it works when you start your console and goes haywire after the box heats up a bit then I'd have to say it's probably a hardware issue.