Hard to say exactly why this is happening.

Here are some ideas:

--? the router/modem might need a refresh (unplug and restart after a few minutes)

--? there are too many incomplete files trying to connect on startup, and this may be swamping the connection. Cleaning out the incomplete folder might work.

--? gremlins. Do the OS X "safe boot" (restart the iBook and hold down the shift key).