There could be a number of reasons, but the most likely one is simply that the Wifi does not have enough power to maintain the connection. You mentioned switching connections and probably got a new modem/router, which might have a less powerfull Wifi. I would try moving the wii closer to test it, to see whether that is the problem.
If that is not the problem, it might be that other devices, or other wireless networks are interfering. Try disconnecting some devices to test. If that doesn't solve it, try changing your wireless channel.