T110 Network freeze then 'No memory modules detected.'

Hi guys,

I currently own a PowerEdge T110 server since a few months.
On arrival, I removed the 2Gb ECC memory stick and put 4 Mushkin 4GB DDR3 non-ECC (I know it's not supported) memory modules (996770).
For a total of 16Gb. It was working well except for the error messages on POST. (memory size may be reduced and thermal sensor not detected for each dimm)

A few weeks after, I needed 2 sticks of ram for another machine so I decided to just removed them (slot 3 and 4)

The system was running with 8Gb (2 x 4Gb) at that time. 

But I started to experienced instability.
Never the server rebooted or crashed. But it was like network freeze.
I was clicking on a folder or trying to open a file and it was not responding for like 10 seconds then it finally appears each time. (OS is Ubuntu Server 10.4)

At 1st, I suspected the onboard NIC. So I added an Intel mini PCI-E card and the problem was still there.
The RAID was working great also so did not point any of my 4 1TB Seagate drives.

Then there is the RAM. I know, it is not Unbuffered ECC, just standard DDR3 ram sticks. But it was working before.

So I decided to try one module at a time by using the slot #1. I then tested the sticks with a memtest86+ bootable usb thumb drive.

But since I tried to switch ram sticks to test the 2nd one, the server is not booting anymore. Only black screen. No Video or POST.

Only 1st diagnostic led lights up. Which means "No memory module detected" according to owner's manual.

I tried many different ram sticks in slot #1 (Mushkin 996770 and Corsair XMS3). None is detected.

I ordered 2 sticks of Kingston Unbuffered ECC DDR3 ram. So will test these soon.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820139262

What I don't understand is why it was working before and why is it not now ?
I didn't update the BIOS. 

Any idea ?

Thanks,
Maxime