PC drops from 1Gbps to 100Mbps

We have a new Dell PC(T3400, 4GB ram, Dual CPU, WinXP x64 SP3)  that randomly drops from 1Gbps to 100Mbps. It is connected to a 3Com 48port Gigabit Ethernet switch.

We have tried:
- changing the PC patch cable, switch patch cable,
- changing switch port,
- changing connection to another 24port GBE switch,
- reinstalling NIC drivers,
- changing the LAN data patch point,
- If I force the managed switch port to be 1Gbps, the PC is unable to connect,

I added in a PCI GBE NIC and the PC connection is back to 1Gbps.

Any suggestions?

Thanks.
artradisAsked:
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geowrianCommented:
It is likely either a defective NIC (they negotiate the highest they can by default, which can change) , or the wiring going to the server (assuming it is not going directly into the switch - if so, this is moot based on what you have tried).
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Arabia_vnCommented:
Maybe the distance of connection is a bit long so the cable you use is not good enough for 1Gbps. Try more expensive cable.
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artradisAuthor Commented:
Arabia,

We are on CAT6 certified cabling. The distance is max 45m to the switch. We have also tried changing LAN data points and no other PC is encountering this.

Cheers
Joo
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artradisAuthor Commented:
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