Unless that switch is multicast capable, your setup is flawed anyway, and adding the PLC just magnifies the problem.

The technical feature you're looking for in a network switch is IGMP/MLD awareness. For TP-Link product, the "Easy Smart" series does support this correctly with the latest firmware, while the plain switches don't.

The background is, if anything in your LAN infrastructure doesn't handle multicast traffic correctly, that traffic ends up getting broadcast to all ports. If one of those ports is substantially slower than the others, things will start to fail.

[As a side note, the 4000 series is a bit weak for IPTV streaming anyhow. I strongly recommend you use a pair of 8030P. By means of these, you will at the same time eliminate the separate switch, replacing it with the 8030P's internal one. This one has IGMP/MLD capability. Cabling would be as follows: Router to 8030P, IPTV1 and IPTV2 on the remaining two LAN ports on 8030P; IPTV3 on the other 8030P wherever that is.]

@Peter: Thank you for your insight.

Curiously my ISP has specifically stated to use a standard unmanaged router with a minimal of features. And honestly with everything cabled it all works perfectly.

Only with the power line attached does it seem to fail.

I will ask Netgear to see if my switch supports IGMP Snooping or not.

To be continued...

I resolved the issue by replacing the TL-PA1016 Kit with a kit from a different vendor. All other components stayed the same. I am now able to view IPTV in full HD on all three STBs.

It's not so much about the router, much more about any extra switches and WLAN access points. Those need to be multicast-aware as well, or else your IPTV traffic will multiply onto everything, not just the one client machine that subscribed onto the multicast stream.

With multicast-unaware infrastructure, things often seem to work well, with really strange triggers for malfunction. In my house, the upstairs IPTV would work perfectly fine (on a 180 Mbit/s powerline connection) until someone pulled out their smartphone in the same room. The WLAN AP would start firing the IPTV stream toward the smartphone, thus overloading the powerline connection, thus congesting the router and ultimately breaking the stream.

After I replaced the downstairs TP-Link "Desktop" LAN switch with an "Easy Smart" model, those problems are gone.