I'm having a serious issue with dell server PE2950. The server has really slow IO rates, so slow that I'm not able to use it anymore

I tried few things to solve this:

1. changing disks to new disks (configured them as raid1)
2. changing perc card + perc cables
3. reinstalling the OS of course, had to cause of changing of disks, centos 5.5 x64bit
4. firmware update to everything
5. virtual disks policy: No Read Ahead,Write Back, disk cache policy disabled.
6. openmanage doesn't alert about anything, also i ran dell's diag tests, everything passed, also dell didn't see anything in deset log.
7. dell offered to reseat everything, including the cpu, we did that as well, still io rates are slow

I have several PE2950 servers, and I never had such a thing with any of those. All have similar or exact hardware as this one, all configured the same, with the same os centos 5.5 x64, same disks, same raid, same policy.

Just for comparison:

the problematic PE2950 server:

[root@bad ~]# time sh -c "dd if=/dev/zero of=/tmp/ddfile bs=8k count=200000 && sync"  

200000+0 records in  
200000+0 records out  
1638400000 bytes (1.6 GB) copied, 27.7946 seconds, 58.9 MB/s  

real    0m33.968s  
user    0m0.531s  
sys     0m26.000s

good PE2950 server (with the exact same hardware):

[root@good ~]# time sh -c "dd if=/dev/zero of=/tmp/ddfile bs=8k count=200000 && sync"  

200000+0 records in  
200000+0 records out  
1638400000 bytes (1.6 GB) copied, 3.19999 seconds, 512 MB/s  

real    0m7.694s  
user    0m0.053s  
sys     0m4.057s 

Hopefully you will have an idea what can cause the problem.