I've just set up a router running on a Debian box with IPTables at work, so that it routes from two external IP's to different PC's and services inside the network. Ever since installing the router (Taking the place of a DLink 604+), all the PC's behind the router on the LAN take about ten seconds to resolve hostnames. If I change the DNS Servers on the LAN PC's to our ISP's external DNS server, hostname lookups are perfect. So the problem must be on the router side. The routers (192.168.0.1) resolv.conf goes like:
nameserver 203.161.xxx.xxx
nameserver 203.161.xxx.xxx

and each LAN PC is setup (standard machines through DHCP):
IP: 192.168.0.x
Netmask: 255.255.255.0
Gateway: 192.168.0.1
DNS: 192.168.0.1

Route table reads:
Destination Gateway Genmask Flags Metric Ref Use Iface
172.31.150.1 * 255.255.255.255 UH 0 0 0 ppp0
192.168.0.0 * 255.255.255.0 U 0 0 0 eth0
default 203.161.103.103 0.0.0.0 UG 0 0 0 ppp0

Can anybody think of any reasons why this might be happening?? Running BIND on the router as a transfer DNS from ISP's maybe?? Any suggestions would be much appreciated. I've googled this, but can't find any answers that work..

Note: LAN PC's can, of course connect and ping a raw IP perfectly fine (so its not a problem with net link, or the routers IP forwarding in general).