Increasing the timeout does help, but the 503 will still come up on large updates...for me it's about 3% when I do more than 30 movies...and the percentage rises up to 10% for several hundred updates. Once the error occurs, the odds are that the next 1 or 2 records will also fail.
I've noted that when you go to IMDB while doing an update, you're likely to get a message about too many requests from your IP. This may or may not be related, but I suppose this might be the cause of the problem. If IMDB sends a completion header, but the server doesn't run quickly enough to update internally, a second request might fail based on the completion status on the server.
I know this has been discussed before, and the internet options (advanced) settings were added in response, but there doesn't appear to be a configurable wait time between requests (only necessary, I think after an error).
Silent mode does hide the errors, but then you have to find the errors somehow and re-run the updates. The advanced search for movies (url not containing imdb) catches many of the failures. People come up as hidden records when added from the movie script, and only become visible after the record has been updated, so filtering on "not visible" works. Your mileage may vary.