SFTP Implementation on .NET Framework

I have a program written in .NET framework (3.5) which downloads files from a SFTP server. I earlier used Chilkat Dll (old version)(9.5.0.66) which worked but now I am facing frequent problems. Some files are downloaded and some fail and I have to retry. Maybe they upgraded SFTP server (I don't know).

Can anyone help me with SFTP library for .NET Framework which will work in older version such as 3.5 or 4? If anyone has latest Chilkat library please let me know.

Thank you