FTP to mikrotik becomes slow suddenly.

Hi everyone ,
I have a x86 system with routeros 3.28 on a 8gb High speed CF card.
When i try to send something to the ftp of my rooter via ethernet 100 mbit, the transfer rate is 80 mbit for 5-40 seconds, and then suddelny it stops responding for about 5 seconds and the CPU stays at 100%. Then it continues at 80mbit again.

The same thing happens at an rb43AH with different CF.

Does anyone know why is this happening???