Do you have phpMyAdmin? Or have any other way to do an sql command? It may be an issue with the data in the database. If you can do an sql call run this query:
PHP Code:
SELECT COUNT(*) AS referrals, user.username, user.userid FROM user AS users
LEFT JOIN user ON (users.referrerid = user.userid)
WHERE users.referrerid <> 0 AND
user.userid NOT IN (0)
GROUP BY users.referrerid
ORDER BY referrals DESC
LIMIT 100
I wouldn't be surprised if you get some weird result, but either way, post results here and we'll take it from there.
BTW, I really like your site design. You have a good eye for design.