OK. This is a bit of a weird problem... And I need to elaborate...
I originally had our network setup, as depicted in the attached file (1.jpg). EVERYTHING was working great.
Well, unfortunately, the Netgear ProSafe firewall had a complete hardware failure, and I ended up having to remove it from the picture. (and now the network is as it's depicted in the 2nd file (2.jpg).
Here's the problem: Web requests are sent to the Web Server (192.168.1.254), and I've got DNS on the Domain Controller that all of the hosted sites are referred to the internal address.
But, since I've lost the firewall and had to move the domain controller into the 192.168.1.254 network, it's begun answering for web requests that are supposed to be destined to the web server.
The DC has two nics, one for the "external" lan, and one for the "internal" one. At (what looks like very random) intervals, the DC starts answering requests for all web traffic (coming from the router) that's supposed to be heading to the webserver.
Now, the really weird part, is that this is ONLY for traffic that's inbound from the router. Anything originating on the internal 188.8.131.52/24 network still works like it should, and the web server answers traffic.
When the DC takes over for answering web requests, the only way to fix it - is to restart the DC - and everything automatically falls into line again, and external users are sent correctly to the web server.
I'm just trying to wrap my head around why it could be doing this. It will all go away shortly, as I've ordered another firewall some time ago, and just waiting for it to get here in the mail. With the new firewall, I'll be able to put the DC in a completely separate network again, and this problem really will be a thing of the past.
Anybody have any ideas why something like this would happen? In IIS on the DC, it's only answering requests on the internal NIC.