Flaky FTP

Here's my situation...I have a process that FTPs a file from a Windows
XP client to an AS/400 (V5R1) every 15 minutes. Runs like a
champ...does everything I want...except it's started getting flaky.
Every 10 or so transfers, it doesn't move the data. Here's the script
that runs on the XP client:

user
pass
quote rcmd addlible mylib
quote rcmd call parsetimex 
append c:\ta\punch.log hravfa/hptimein 
quote rcmd call parsetimec 
quit 

I've been running comm traces on the AS/400 and Ethereal on the XP
client. When it fails, I don't see the append command; when it works, I
do. It's almost like it doesn't recognize the append command when it
fails. I'm baffled...any ideas? Or better ways of doing something like
this?

TIA...