what i had to do....
when i recieved my mac book pro i had to turn off the air port, unplug both the main internet (dsl unit or comcast) and the wireless unit, and then plug them back in, and as soon as they both came back up i turned the air port back on. Since then my internet has been just as fast as it was on the eathernet. Anyways, thats what worked for me, id say atleast give it a shot...
Tim