Tell me about it. I've been having a horrible time with my MB. I can be sitting on top of my AP and have 1 bar and a 1Mb connection. Absolutely unacceptable. Going to the Apple store again about it when I get a chance.
This is often a hardware problem in the computer (loose antenna, being the common source) or a problem with the base station. Try changing your network to a different channel and see if that helps, otherwise, I'd get a Genius Bar appointment.