Could be a lot of things, but you mention "built in" which makes me think the built in wireless cards are Intel based. If that's the case, try updating their drivers to the latest version. Intel has been slowly improving the quality of their drivers, but has long had issues relating to signal strength/reception. With each driver revision, the issue gets better and better, but in the end, avoid anything with the Intel 2200 B/G chipset (most centrino laptops more than a year old).
