First off, if you have a Homehub5, can you post your line stats from the HH5 please? These are in Troubleshooting/Helpdesk on the web interface at http://192.168.1.254
Lines 1-13 will be enough
The line stats may help to answer your question.
Sorry, I can't help you with a fritzbox. If you have a HH5, I can help.