Portion of XML files on FTP is missing

We have a very strange problem. I can open WinSCP (V 6.5.5) in Commander mode and go to a standard FTP (port 21). There are about 40 XNL files in the folder. When I double-click on a file to view it, the file has about a 90% chance of showing complete. About 10% of the time, part of the file at the beginning is missing, and sometimes part of the file at the end is missing. If I click on another file and come back to the offending file, it will often open in its entirety. If I download a file to my hard drive, it also has about a 90% chance of being complete. This behavior is shown throughout the ftp site. I have tried clearing caches, uninstalling and reinstalling Win SCP, and nothing has fixed this. I should also say that our ERP is erroring out when trying to read the files, so it is experiencing the same issue. The vendor who is providing the FTP and is placing the files on the FTP swears they do not see the problem at their end. Any ideas would be greatly appreciated!!