Even though Rebex SFTP 2.0.4086 is more than 6 years old, we are not aware of any issue resembling what you describe. What do you mean by "heavy load"? Are you calling GetFile multiple times at once? In any case, a communication log at LogLevel.Verbose makes it possible to inspect the raw content of SFTP data packets, which could be used to determine whether it's correct.
In any case, give the latest version a try. The API stayed almost the same, but the internals have been enhanced a lot.