i'm assuming that your macbook has leopard on it, if it's recently purchased.
next time you notice the internet is slow, try cycling your airport on/off (go to the airport icon in the menu bar, turn off, wait a few seconds, turn on - or do the same from system prefs.)
for my macbook this will often work. i have noticed that my macbook has difficulty in re-establishing a connection after sleep, so every time i open thie lid, it "sort-of" connects. (ie. it says its connected at full strength, but the speed is horrible.) cycling the airport fixes this issue for me.
never had this issue on a g4 powerbook running tiger, so I dont know if its an intel issue, or a leopard issue.
let me know how it goes!