Need to go thru the house and make sure all wireless devices are off. Turn off everything you can accept for one pc, try both wired and wireless. Then try a different PC and test again. Graduate adding devices. Might reveal something.
Might check out INSIDDer application as well as you might be getting some external interference from other WiFi routers near by.
Sounds like this is interference, just need to isolate it and get rid of it. Keep us posted.