Every night, for about 10 minutes, the connections from our HQ to a relay tower became flaky. At the time we were using two Mikrotik 5GHz cards and some large antennas.

You could sit in front your computer and wait, a few minutes after the sunset, for the monitoring alerts start arriving. After a week trying everything, including changing hardware (to the same specs), I was very disappointed with the thing and got out to smoke around the sunset.

Then some huge lamps we had around the building switched on, based on a light sensor. Immediately I received the SMS alerts on my phone. I ran into the building, turned off the external lights and bingo: 0% packet loss.

It turns out that the building management had changed all of external lamps the week before, with new sodium-vapor bulbs. And for some reason, on the first 5 to 10 minutes with these lights on, it caused very high interference on the 5GHz band.

Changed the lamps, problem solved.