Unstable LAN connection.
With age, my Zyxel 660-HW started to be instable on all 4 LAN connections. All other functions were fine.
Symptoms were: poor LAN performances and all LAN connections lost and soon reestablshed every 15 seconds or so.

The cause was the electrolytic CAP highlighted red in the following picture.
Replacing that cap cured the problem.

Most of the electrolytics measured bad ESR values. In particular a couple in the power supply (marked in yellow in the previous picture).
Shouldn't you have a chance to measure ESR, replace all CAPS. 

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http://xoomer.virgilio.it/egirland/zyxel-660HW/repair%20brief.html