This could be a clue,

One of our customers in London has 2 offices in 2 buidings that are network connected by 2 dishes pointing at each other, (don't know what sort of link it is, possibly microwave?). Earlier this year they would lose connection every now and again at exactly the same time of day from one office to the main network in the other building. After a LOT of work they discovered it was the sun shining at a certain angle into one of the dishes that was causing the loss of comms. Obviously on a cloudy day it didn't happen, and when it did happen it could last up to 15 minutes or until a cloud passed by! That's all the details I can offer, but they cured it by making a permanent shadow on the one dish!