Hi, I'm new to this forum, so please forgive me if I don't follow usual procedures.
I had the same issue with my Fritzbox 7530 (1st version) constantly rebooting directly after I updated the OS from v7.59 to v8.00 a week or so ago. I soon gave up with Zen tech. support when it became clear to me that they didn't have a clue.
I had been reading the messages in this forum and I began to suspect that the issue is caused by a faulty piece of firmware which controls the DSL section of the router.
(I came to this conclusion because the issues only appeared to be affecting Zen users in this country.
The UK are one of the last to switch away from DSL, so that would fit in with European users not being affected by the rebooting issue.
To test this theory I bought a cheap TP-Link TD-W9970 VDSL router from Amazon (£23.49) and set it up in Modem Only mode (Bridge mode) connecting it to the Openreach DSL line instead of the Fritzbox. Then I setup the Fritzbox in Router Only mode and connected LAN1 on the TP-Link router to LAN1 on the fritzbox with an ethernet cable. It's been working perfectly ever since (3 days now).
See these 2 videos I found on Youtube for how to do it (It is really simple):
https://www.youtube.com/watch?v=CbN1apeTbLI
https://www.youtube.com/watch?v=1l1KgEh4kHU
I'll probably just leave it like that until I switch to FTTP in the future (All I'll have to do then is connect the Fritzbox to the new Openreach FTTP modem)
I hope that helps someone