It’s been a while back when I did this, but there used to be a ‘max’ setting for file import size and upload size.
The size of the import was controlled by the maximum execution time (set in php.ini), also.
You might look at those, increasing them one at a time, until you find the variable that allows for bigger imports.
Another possibility is that you have ‘gremlins’ in your email list that you are importing (unprintable characters that don’t show up in html…). The best way to find these hidden characters is to look at the file using a text editor such as textmate or vi. If you get stuck in the same place (i.e 3300th email address), you can look at that line and make sure it is free of weird looking charachters.