Low data rate

Hello!

I'm use FreeNAS 9.2.1.7 x32

Gigabit Ethernet D-link DGE-528T
Intel Pentium IV 3.0 GHz
RAM 1024 GB ( 2 x 512 )
Data HDD: ST31000340AS ( 1.0 TB ) SATA
System HDD: WD400JB (40 GB) IDE

When I start to download a large file (about few gigabytes), the speed reaches 40-60 MB s, but a few minutes later drops sharply to almost zero (about 50-100 kB/s). And does not return to previous values until the system reboots.

I have this problem on FTP and CIFS (SMB)..

1000 MB LED indicator on...

Low data rate