Question : Windows 2008 Standard server connects at 1GB/S but reverts back to 10MB/S

Hello, I have a Windows 2008 (64x) E-mail server with an on-board Intel 82567LM GB Network connection.  The connection upon reboot connects at 1GB/s and effectively runs and transfers files at expected speed.  

However, sometime during the day the connection "slows" down to 10MB/s and does not revert back to 1GB/s until reboot.

We have attempted changing the switch, cabling, using a different NIC (from the same motherboard) with no luck.  They all start out at 1GB then slow down to 10MB.

The current motherboard is a S5500BC.

I'm currently stumped and any help would be greatly appreciated.

Answer : Windows 2008 Standard server connects at 1GB/S but reverts back to 10MB/S

We had to replace the motherboard.