Year ago I picked up an RJ45 crimping tool and have been making my own Ethernet cables periodically ever since. I'm definitely not as adept as a pro who assembles these things all the time though, so I probably untwist the pairs a bit too far and strip a bit too much shielding, etc. On a less related note I also occasionally force the cables around a tight opening where they get crushed, bent, etc. The point I'm driving at: all of this makes me wonder, with Ethernet cables getting ever faster and more sensitive, do they generally function in a binary sense, i.e. either the cable works and you get a good connection, or else you get no connection? Or can a poorly constructed or over-worn cable still present as though it's making a good connection, while in fact your signal quality / bandwidth is adversely affected?
The reported "link rate" will always be fixed to a certain standard mode, by default the highest supported mode between both devices. The only choices supported by typical Cat5 Ethernet devices are 10/100/1000 Mbps – therefore, unlike Wi-Fi (which has a large table of possible link rates to choose from), you'll never see Ethernet devices negotiate e.g. "300 Mbps" or "54 Mbps" modes.
Normally the negotiated "link rate" stays fixed. It might work, it might work poorly, it might stop working, but the rate does not change dynamically over the duration of the link.
However, even though the interface shows "Link: 1 Gbps" all the time, it's possible that the actual ratio of data successfully transmitted vs lost can vary. As far as I know, "working poorly" means packets are more likely to arrive corrupted and get rejected by the receiver, which TCP interprets as packet loss and waits for the server to retransmit the data, which results in lower transfer speed overall (even more so due to TCP assuming the loss is due to congestion and deliberately slowing down). Tools like
netstat -s (Windows) can show you the number of "TCP segments retransmitted".
(And if the connection is really poor, at least some Ethernet devices will reset the link and re-negotiate with a lower mode once, e.g. I've seen 1 Gbps-capable devices switch down to 100 Mbps mode after a short while, and likewise 100 Mbps ports re-negotiating 10 Mbps. I don't know whether this behavior is standard.)
This could be better answered by an actual Ethernet installer, which I am not...