I've had a good search through the forums and standard troubleshooting, but what I appear to have here is anything but a standard issue. As the title says, we have a laptop which runs fine when connecting via wireless, but when connected by ethernet cable it gets constant disconnections. There is a second system (desktop) which does not have wireless, connecting by ethernet also has constant disconnections. That system has been tested using mobile tethering, at which point it had no problems.
A different system entirely on a different connection using the same model router produces the same results - constant disconnections. Both connections run over fibre, using a single router to provide a PPPoE connection. This router has N wireless, VOIP and gigabit LAN capabilities. As the wireless network is running absolutely fine, to me it's pointing towards LAN issues with how the router is directing the traffic. A different router model has been tested on the same connection, and there were no issues. The main difference between the two is the LAN is not gigabit on the working one. The next step I am considering is limiting the LAN network to 100mbps, to see if that makes a difference.
Any other suggestions would be welcome.