Well, the problem is that wireless is a very finicky communication system by its very nature. There are a large number of environmental factors that could be at work here. So probably your best bet, if it's still under warranty, is to take it to an AASP or Apple store and see if they will swap out the airport card.

Also, you have an Mid-2009 model, there was no Late 2009 model for anything but the iMac and MacBook. After that came the Mid-2010 models, and they just released the Early 2011 models. Doesn't make a huge difference, but better to look a little foolish in front of faceless strangers on a website than in front of an actual person.