Matty,
I have the same sort of set up....... The DHCP at the Cable sups. end is the hassle... you must be on a busier segment than me, mine rarely changes IP despite the 4 hour rule.... but use the ipconfig /renew method occasionally....

As an Aside, put ZA on a friends laptop...they use the 'Dial in from anywhere' AOL service, and during the log on phase ZA will block the handover on protocols, failing the connection. Only way ive found to get round it is to remove AOL each time and then re-connect. anyone else seen this, or know of the best way to resolve?