I laid a 75-foot length of Cat5e cable from my Linksys WRT350N router to my media center PC I just built using Windows 7. I get only 100mbs speed even though the same PC connected to the same port with a commercial 12-foot patch cable gives me 1.0Gbs speed. I get only 10.0 mbs with my son's laptop running Vista.
I had attached the RJ-45 connectors to each end myself so I thought maybe I didn't do so good a job since I'm kind-of new at this. (Although two other connectors I installed for two other ports on the router - each about 6-ft long supply 1.0Gbs.)
I tried then to use a wall plate on each end of the cable and use 6-ft patch cables. This did not help. There are no electrical wires going parallel to the network cable. There is one place where it crosses electrical wires and a 10-12 ft length that runs close to a speaker wire but the speaker is not connected to anything.
Anyone have any suggestions? Are all 8 wires in a Cat5e cable in use? I had a similar problem with the phone wire going from the telco box to my router that I fixed by switching from using the blue/blue-white to the green/green-white. Those wires are so thin it is conceivable one may have broken while threading the 75-foot length through walls etc.
One more thing, my problem is not just speed. The Windows 7 machine has trouble identifying the network and internet speed is extremely slow.
It turns out I was in fact running parallel to (actually right on top of) a romex electrical wire for about 20 feet. I was able to move the cat5e to about 4 inches (best I could do) from the electrical wire and that completely fixed all my issues!
I had no idea network cable was so sensitive!
Regarding the statement you made about all 8 conductors being used, that conflicts with other information I got from my IT guy at work. He says there's only two pairs in use: one for receive & one for transmit. Are you sure about all 8 conductors?
1000BASE-T requires all four pairs to be present and is far less tolerant of poorly installed wiring than 100BASE-TX.
If two Gigabit devices are connected through a non-compliant Cat5 cable with four pairs, many FCS errors and retransmissions may occur. If two Gigabit devices are connected through a non-compliant Cat5 cable with two pairs only, negotiation takes place on two pairs only, so the devices successfully choose 'gigabit' as the Highest Common Denominator (HCD), but the link never goes up. Most gigabit physical devices have a specific register to diagnose this behaviour. Some drivers offer an "Ethernet@Wirespeed" option where this situation leads to a slower yet functional connection.