1. Router defaults.
2. WPA security.
3. Change channels.
It's that simple and has helped many.
I have a Linksys router on my downstairs home desktop computer. I also have an upstairs desktop computer with a wireless antenna that picks up the WIFI signal. For about a year the connection was perfect. Now, for the past few months, the connection is choppy. It's great for a few minutes, then cuts off, and then great and cuts off - and so forth.