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I agree with @gvdhoorn about testing with another source of traffic. I suspect that you're seeing issues with noise or attenuation once the system gets loaded.

If you're having trouble with the 1' cable I'd start by upgrading that cable. All of these systems are packet forwarding so signal degredation does not pass through devices like a switch. So you can focus your debugging on just the problematic link since you have an indication localizing the problem.

After that I'd look at upgrading the switch. The transition to "managed" is not the right metric. You want to be choosing a switch with a higher quality electronics. If you pick your netgear or similar routers carefully they can have very good performance as long as they didn't apply too many "cost" saving measures. Make sure the switch you're using has enough backplane to fully isolate and switch gigabit between all ports independently in parallel.

Lastly unfortunately the other place that you might be running into a problem is your computer's NIC. It's possibly got buggy firmware or it's driver is not keeping up under load such that it misses deadlines and the link gets negotiated down to a slower speed. If you have another ethernet port I'd suggest trying it out. Or if you have space try an expansion board to supply an extra ethernet port on the PC.