The first thing I did was to reset the adapters to factory defaults, but with no joy. So I changed the network cable, a favourite pastime, but again no luck. Then when I was plugging in another cable I spotted something odd in the network port on one of the adaptors. The gold sprung contacts didn't seem regular, and on closer inspection one of them appeared bent. I tried wiggling it with a fine screwdriver but it didn't want to budge. Sure enough, wiggling the network cable during a test allowed me to recreate the problem on demand.
|The far pin was the culprit|
|Bent or damaged pins do stand out. But they are difficult to fix.|
We take network ports for granted, but they are very easily damaged - I have no idea how this happened, but as powerline adapters tend to get left lying around, it was most likely to have been a foreign object, possibly attached to a small human being's hand.
Checking out the network connector pinout diagram here, I discovered that it was Pin 8 that was damaged. This is interesting, as for 10/100Mbps Ethernet, this pin is unused. But for Gigabit Ethernet it carries the BI_DD- signal, apparently. So if this was intermittent, the connection would keep switching between 100Mbps and 1000Mbps, which would neatly explain the performance difference, as the lower rate would be maxing out a 100Mbps connection.
The moral is, if you have network performance problems, or you can only get 100Mbps out of a 1000Mbps network device, it's always worth checking the ports themselves (there are other causes, however). It only takes a second, and bent, misaligned or damaged connectors really do stand out a mile when you look.
On a side note, it also highlights why, whenever running performance tests, you should always repeat them several times, something constantly drummed into me during my PC Mag Labs years.