Large ftp file times out

We're having a mysterious problem with one of our servers that is hosted at
a large ISP company.
We have a nightly batch ftp process that copies a large file from the server
to a server in our
office. We use a scheduled W2K command prompt ftp to do the work.
The problem is that the file is not always copied successfully and we can't
figure out why. The files are around 70 MB. It appears that a connection is
lost sometimes.
The Internet connection at the ISP is big, but the connection in the office
is business DSL.

Do you have any advice? Thank you.