ProFTPD: 421 No Transfer Timeout (300 Seconds): Closing Control Connection

Since moving servers I've been plagued by constant disconnects whilst using FTP:

421 No transfer timeout (300 seconds): closing control connection

I've gone through Proftpd's forum and documentation numerous times to try and find a solution to this but have been unsuccessful so far.

Within proftpd's config file it's set at:

TimeoutLogin 120
TimeoutIdle 3600
TimeoutNoTransfer 3600
TimeoutStalled 3600

So I am unsure where it is getting the "300 seconds" from.

Even though the error states 300 seconds, this problem happens way before 300 seconds every time and has happened during the transfer of files (when the connection has been active and in use).

I've tried 3 different FTP clients and used the "Keep Alive" option in each and it has absolutely no effect.

I am unsure if APF is causing the problem, I can't see any problems in any of the server logs in relation to ftp.