I'm running Centos with Kloxo CP and PureFTPd on VPS based on HyperVM. The problem - during the day, there are periods when FTP wors fine for hours, but sometimes it dies with "500 Unknown command" code. for files larger than 2Kb.

This is really weird - with files less than 2472-2484 bytes everything would work fine, but with bigger files ftp would start writing file, would drop on 2Kb and would return "500 Unknown command" .

This happens on Passive AND/OR Active connections for any filetypes.

Logs are showing like the file was uploaded successfully , but the speed nearby the action would be 0.3 -0.5 Kb , when normal operation would report 70-150Kb/s speeds.

I do have some Iptables firewalls on both HyperVM host,and VPS , however these are static, and most of the time it works just fine.

Any suggestions on where to look for a solution would be appreciated :)