SFTP file upload issue

We are working with one of our customer to resolve an issue sending files to a SFTP server on their network.  We're using the SFTP module version 2.4 in our application.  The customer's firewall has been configured to allow the Mendix Cloud public IPs.  We are able to use the Explorer option in the SFTP module to connect to the server, remove files, create directories and change directories without any issues.  However when we  try to upload/put a file in most cases the upload fails.  Part of the file is uploaded but the session hangs and eventually will error with a timeout error.  On occasion (maybe 5% of the time), the file will upload successfully.  We've tested this with multiple file types of different sizes with the same results, and this happens in all of our environments (Test/ACCP/PROD).  We've also setup one of our own test SFTP servers in the configuration and we able to perform all of the functions (put, CD, delete etc..) without any issue. Our customer has had their server admins troubleshoot on their end, and they are seeing the connections on their server.  They are telling me however that during a PUT operation they see the connections and then see  > 0 bytes in the Send-Q for the  connection.  They say it appears that their server is sending packets to our client but the client isn't acknowledging that request, and therefore the session hangs and times out. Our customer is able to connect to their SFTP server from one of their SFTP clients and perform all SFTP functions without any issue, so it seems like there may be a network related issue somewhere.     Has anyone else seen a similar issue?  
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