FTP Session times out

All,

Several years ago I wrote a program to distribute AS/400 files to a Windows
NT server so that users can review reports as desired. The FTP program
worked like a champ for years. Suddenly yesterday afternoon my FTP job began
failing. The FTP script starts, it sends the file name to the Windows
machine, and then times out, leaving a zero byte file on the Windows system.
I ended the program last night, hoping that the nightly clean-ups (re-IPLs)
on both machines would reset the system. Sadly, the problem stuck around
this morning for me.

I cannot manually FTP a file from the 400 to the NT server. The only log I
can find on the NT server is a timeout message for 900 seconds of
inactivity. 

Funny thing is that at midnight when the system came back up, it
successfully FTP-ed an activity log to the machine, and then again around 8
am or so. Then, just like yesterday, it stopped sending the files. I was
able to send them to another server, but I can't to the primary. Files are
all about the same size.

Anyone have any hints that I could use? It is probably on the NT end, but I
can't find the correct setting to resolve this, and...well you know the
drill!

Thanks in advance,

Patrick