Winscp Drops file transfer on XO but not Earthlink

Winscp drops a large (3 meg) file transfer under ISP XO Communications. Same file transfers just fine via Earthlink ISP. Contacted XO communications. They had no idea why WINSCP would not work. Same laptop will transfer at Earthlink ISP, but will not via the XO Communications ISP... Anyone have ANY idea what is wrong?

What exactly happens? Is connection lost? Does it happen with every file? Upload/download? Can you try to transfer same file with PSFTP client?