Internet speed capped at 1.2MB/s

[ProdigyThirteen]

Lets try this a third time.

This issue only occurs with Gigabyte motherboards using one specific Ethernet cable. If I physically move my desktop to another room and use another Ethernet cable, I'm fully capable of achieving 20MB/s+ speeds, verified by the fact that I have just been doing it today.

Just to clarify;
This has nothing to do with software. My router has nothing to do with this, nor do drivers, speed/duplex or any of the other frequently assumed issues.
The cable is a custom length (under 100m) Cat5e running to my room.
The cable is in perfectly working order as I can access the internet. Each of the pins and wires are correctly lined up.
The cable's connection was verified using a Network Cable Tester


If that's not puzzling enough, here's another layer of complexity;
The use of PCI-e and/or USB 3.0 Network cards and/or the use of an Ethernet plug socket/power-line adapter does nothing to change this fact. I've even tried wireless which gives a higher download speed, this time limited by the physical obstacles between the two locations.

 

[nobspls]

The assumption that your ethernet cable in question is working perfectly is clearly incorrect. The cable tester is is only testing to make sure there is electrical conductivity, and that your pins are more or less correct. However if does not really test for things like wire gauge, signal quality, interference etc.

 

[bill001g]

You pretty much proved it is the ethenet cable by moving the pc. As mentioned above you likely did not actually test the cable. Testers used to certify cable to really pass data cost many hundreds if not thousands of dollars.

So first verify that the cable is in fact cat5e cable. Used to be you could look for eia/tia markings but there is a lot of fake cable on the market. You need to be sure the cable is pure copper (ie not CCA) and has wire gauge between 22-24. This information should be printed on the cable but you can tell by look at the wires also.

After that it is blindly replace one of the ends and hope to get lucky. I would consider using keystone jacks rather than rj45 plugs. It tends to be simpler for a beginner to get the wire correct because you do them 1 at time rather hoping to crimp 8 and nothing slip.