Dsl internet drops couple of times a day

I work for a company & their internet was dropping about 5 times a day.
It only started when a 2nd company with 10 new users began using their same dsl connection.

Rebooting the DSL P-660 Zyxel router always sorts it.
The router has wireless & looks after the DHCP.
There are also 2 other wireless networks which worked well before Company B arrived.
We have replaced the router & replaced the internal DSL wiring but it's
still happening but not as much.
I checked it over the weekend & the router didnt drop but the office was empty.
It was permanently online so it must be an internal issue.
This morning as soon as the lads came in, it dropped again.
I'm thinking there could be a device flooding the network with requests?
There are all kinds of devices connected, windows pc's, laptops, phone's, ipads etc.

I keep getting this warning in the logs. It could be a clue?

Sep 24 13:09:18 ZyXEL user.warn syslog: Warn: The origin for route 239.255.255.250 changed from 192.168.3.57 to 192.168.3.21


I could seperate the networks but Company B will be gone in a month.
I disabled wireless on all the laptops which have wired connections.
I replaced the router with a new Zyxel D1000
It is an OEM.
http://broadbandsupport.eircom.net/download/zyxel/eircom_D1000.pdf

If this router isnt suitable can you recommend one?
Approx 15 users, approx 25 devices connected phones, laptops, ipads etc

another thing that happens is that i cant login to the router via the browser.
they are currently online.
i can ping it ok. only option is to reboot the router & then i can login via google chrome at it's ip of 192.168.3.1.
this also happened with the last 2 routers!
something is hanging it for a few hours before it crashes & needs to be manually rebooted.
crangle22Asked:
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Dirk MareSystems Engineer (Acting IT Manager)Commented:
it could be a rouge DHCP server (AP, router, laptop or printer)
basically any device that can serve as a DHCP server.

when your internet drops, is your gateway still the same as it was when it worked?
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crangle22Author Commented:
Hi Dirk,

I opened this question again in error!
I didnt realise there was a "request attention" option.
This is the original one!
Hopefully you can have a look.

http://www.experts-exchange.com/Networking/Broadband/Q_28250864.html
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