Before on the G4 I had that problem. I don't know how it got solved (maybe when I put it on a different disc?). At some point, it was downloading maps, but not saving them on the disc. After a while, it did save them but download was slow. Then all of a sudden, it is like I put the G4 on steroids, Downloads were blazingly fast.
Now that I put the game on G5, the download is screwed=up again. I repaired permissions, checked the folders permissions and set them to read & write. I don't know how to make it work.