Interesting. You are the first Mac person I've run across that has had this problem. Until now, I had assumed it was exclusively a "iTunes on Windows" issue. Just curious--what type of Mac do you have? Is it Intel-based? Also, can you verify a few other questions? We should be able to figure this out.
1) What is the path of the iTunes temporary directory on Mac OSX 10.5?
2) Did you erase the entire Download directory or just the corrupted partial download?
3) If you erased the directory, did it properly recreate itself after you retriggered the download? Can you still download other things? (You could try the "Free Single of the Week".)
4) After you erased the file and went back into iTunes to trigger the download, does the file start redownloading from 0% (e.g. moving slowly as it builds back up?) Or does fairly quickly skip right back to the place where it is stuck?
5) How much free disk space do you have on your Mac? (I'm guessing this isn't the problem since you were able to download a subsequent episode.)
Answers to these questions will help, but if I had to guess now without them, it sounds like you still may not have erased the corrupted partial download yet, and if that is the case, would explain why the download keeps getting hung at the same point.
If you are desperate to resolve the issue sooner rather than later, if you still have one of your five iTunes computer authorizations available, you could authorize a different computer, download the show, sync it to your iPod, and then use the option in iTunes to resync it back from your iPod into your primary computer. (If you need help with that, search for "Moving iTunes Store purchases to another computer" in the iTunes help.) Good luck!