Hi,
As per the NAT translations the IP 10.1.1.50 is doing more and it seems doing more tranlsation with port 21 (FTP) which is causing to high cpu.
Please trace the above IP and if possible remove it from network or check why the user using doing more translations.
Once you controle that IP your cpu will get down.
If this answered you please click on correct answer.
Regards,
Naidu.