Strange situation with P400 and SATA (slow perfomance)

Hello!

Here is DL 380 G5 serve with :

1. SAS 75gb ( system and some data )

2. SSD sata hdd, 60 gb

3. SATA 320GB Hitachi desk star

All of this running under CentOS

The problem is

1. When I copy files ( > 1GB) from SAS to SSD it works wery fine

2. When  I copy files (>1 GB) from SSD to SAS it`s also good

3. If I copy from any devices to SATA 320 firstly good speed ( 150  - 100 mb/sec) and then it degrade to 30 - 35 mb, and after some time can be 10 mb/sec.

If I put this disk to workstation and copy beetwean disks, everythink is OK. Reads from SATA 320 also OK.

I have tuned P400 with cli utility and enable all caches  ( 25/75 also), but effect same. And writes to SSD SATA moves everytime with same speed.

Any coments ? )

Thank to all