probably is because there's a symbol that is interfering with your data. For example let's say that your file is a comma separated file and one of your records contains this string:
1, text, text, 'some text ' some more text'
In this example the single quotes are used for grouping text that contains spaces. However, there's another single quote that is actually part of the string. This will make your script to stop loading.
This is just an example, but in your case it could be something like this. Check if there's something strange in the record where your text file stops loading,
I think Jamie is on the right track. It's usually improperly escaped characters. In a CSV the double " is used to escape commas. " is also used as symbol for inches. Several times I have seen descriptions like
cause a problem if the " is not escaped as well.
The data problem is always on the line where it stops, it may be a few lines behind. Try deleting 1000 lines prior to the failure. If that fixes it you can begin narrowing it down.