I had same problem and resetting my modem and wireless router worked perfectly. Just unplug both of em for about a minute and then plug em back in. I called Comcast to verify the signal to my setup was acceptable too. If they find the signal is weak, they should offer an amplifier to beef the signal up some.
Finally, if your internet provider is garbage, cancel it, get a Blackberry with a data plan and use that for a modem. I use my blackberry alot at hotels in hick-towns where the wireless is garbage.