D-Link DGE-528T Gigabit Ethernet Adapter only reads 100 mb & not 1 gbps

Hi,
I've a D-Link DGE-528T Gigabit Ethernet Adapter PCI.
Connected to Dell OptiPlex GX620.
Built-in NIC is disabled from bios setup for the machine.
Running on Windows 8.1 Pro WMC 32x with 3 GB Ram.
I am using CAT.6 cable connected to a D-Link Gigabit Switch.
There are another 4 identical machine in the local network and all are reading 1 Gbps except this one.
Cables are all the same for all machines and same copy of windows with exact image using arconis true image.
I've tried all trouble shooting using google but no way out.
I updated
These are the settings from device manager for this device:
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So please help as this is very urgent for business purposes especially that vendor says it is working and nothing wrong with it.
Thanks a lot.