I have this problem wherever there is the Zyxel prestige 600 router installed. This is a very crappy router, hangs everytime we switch to inverter poewer and sometimes for no reason at all. (and no ups or regulator will fix it) it seems similar connections using the older Sisco routers work much better, although there are occasional failed lookups (niothing a quick F5 won't fix)
Since Codetel refused to change my router I am dumping their corporate DSL service in favor of Tricom's. (I used it for years with no trouble) Can't wait for Tricom to install the DSL, internet is almost unuseable at the office.
A little web searching indicates that the Zyxel 600 is a pretty unpopular little box, and there is no support from Zyxel I could find, at least not for the North American region. Either they dropped it or they only export it to 3rd world countries and don't care whether it works or not.