[Hampshire] [OT} weird web behaviour

Hi Folks.
Connecting a Mac Book to my home wireless network. The IPs of all the
clients are static and the ISP DNS Servers are configured "in all the
right places". However... a select few websites will not load unless
DHCP is enabled (for the LAN) on the router just downstream of the cable
modem (I'm in the US) and the mac book uses DHCP to obtain its IP address.

Why is this, anybody know?

TIA
Mike