Wierd file xfer problem in XP

I have remote units that transmit data to central hq every week via FTP. It seems that some of our units connect and begin the xmit and then it quits. Can xmit files ok if they are under 1100 bytes, any more and it will create the filename and quit.The only thing common is they all have WinXP Pro but all WXPPro don’t have the problem. It tends to be on DSL circuits but happens on some dialup. SBC Yahoo DSL doesn’t xmit at all but SBCY dial does??? Things I’ve tried.
Set up another FTP server on the net not behind a firewall–no go
Not use VPN–no go
Set MTU to correct setting–no go
Have them disable DSL and dial directly to my RAS–WORKS
Any ideas???