Greetings,
What is the NIC connected to? Is that device 10/100 or GBE? Cable you are connecting with needs to be Cat5 or above.
Did you install intel chipset .inf drivers?
Did you install the LAN Optimizer Utility add on? If so, I suggest you uninstall that, restart and retest.
Since it was working previously at 100mbps, its doubtful its a cable or port. Most likely related to software.
Look at the properties for the adapter, make one change at a time, then disconnect / reconnect the cable after each change to see if negotiation changes.