Can you diagnose this network performance issue?

We recently were asked to look at a client's network because it is performing poorly. This report shows the quality of the network for 30 days:

network performance

The report clearly shows that the network is experiencing packet loss (red graph), which is causing network quality score (light blue graph) to deteriorate. The uplift in delay (dark blue graph) resulted from the introduction of an internationally located probe that has allowed us to provide triangulated link comparison, and is therefore not related to the network problem.

We found it interesting that the period of poor performance lasted about 12 days. The bottom part of the report shows network utilisation, and it is quite apparent that the problem with the network is not related to how the network is being used. It is also apparent that the pattern of packet loss does not follow a usually daily work pattern. By using triangulated link comparison we can determine which network link has the problem, and we can determine that the problem is on the WAN and not the LAN. The ISP is now investigating.

Do you have any suggestions what the cause might be? If you know anyone who is inspired by these sorts of challenges, please forward it to them.