In my apartment building there are around 30 persons sharing the same modem/internet connection (500 / 50 Mbit). There is one modem and a router with several access points connected via ethernet.
90% of the time, everything works well. However, at some random times (seemingly) most of the users loose their internet connection completely or the internet speed drops to unusable. This applies to both to devices connected using WiFi and ethernet.
The strange thing is - "older" devices still works perfectly. For instance, I have a desktop computer with two NICs. One port has the Intel I211 ethernet controller (released Q4'12), while the other port has the newer Intel I219-V (released Q2'15). Everything works fine using the I211 port, but if I switch to the I219-port, there is a connection, but no internet access. Same problem with the WiFi adapter.
No newer smart phones, tablets or laptops are working. Some old laptops (at least two old ~2010 ThinkPads) and some older apple tablets works perfectly.
The problem usually last for a few of hours before everything suddenly gets back to normal. Router/modem/accesspoint reboot has no effect. Connecting to a different access point or switching to 5 GHz has also no effect.
I have been searching around alot to find answers to this problem. I have been looking into WiFi interference, but I don't see how this can affect ethernet devices. We have contacted the owner of the building several times and he has been in contact with his networking guy several times, and also the ISP. According to the ISP the connection to our house has no bad indications during the "downtime". However, a few weeks ago the ISP did some more searching and found an error one of the cables leading to our house. They have now dug it up and should have fixed it, but the problem is still here...
Any ideas what could cause this problem?