precision T3500 crashes

Hallo ,

We run into big problem with DELL Precision T3500 workstations .

Our company have got 20 workstations equipped with :

Xeon X5550 CPU's
12 G of RAM
SAS1068E controller
and Quadro FX 1800 nvidia card .
The problem is that they randomly crashes ,
some of them have logs in BIOS like:

Uncorrectable ECC Error SMBIOS Handle=   1105 DIMM6

others just silently crashing after they have been running more then 
2-60 days , I understand that it sounds funny , but that is the way it is .
For example I have workstations which had uptime more than 30 days ,
 and than it started to crash 3 times per day ....
I don't think that it is a RAM error , because there are at least  14 
workstations which crash with such message ,
 exchange of modules doesn't help.
I have upgraded BIOS to the last revision, but it also didn't help , 
they still keep crashing .
We use Debian and Ubuntu distros of Linux and Windows XP as well .
on linux systems computers usually  hung , no ping , no keyboard 
actions , sometimes they just reboot with message about faulty memory module .
on Windows XP ( installed from  dell cd ) in some 
cases shows up the blue screen  with message:
*** Hardware Malfunction
Call your hardware vendor for support
*** The system has halted ***
the same from Linux kernel looks like that :
[10169.455422] Uhhuh , NMI received for unknown reason 20.
[10169.455422]
[10169.455422] HARDWARE ERROR
[10169.455422] CPU 4: Machine Check Exception: 4 Bank5: 0000000000000000
[10169.455422] TSC 0
[10169.455422] This is not a software problem!
[10169.455422] Run through mcelog –ascii to decode and contact your 
hardware vendor.
[10169.455422] Kernel panic – not syncing: Machine check
[10169.455422] Do you have a strange power saving mode enabled ?
[10169.455422] Dazed and confused, but trying to continue.

Does anybody of you have seen something like that ? Could it be that 
this series of Precisions  are completely faulty ?