Okies:
1) Where abouts in the country are you?
2) Is your TV service affected in any way?
3) Does resetting the STB ALWAYS fix the issue?
4) Are the dropouts at regular intervals or are they random?
I find it unlikely that the firewall that you have had for so long would suddenly decide it wanted to block DHCP renewal, it's either a problem with ntl's DHCP servers (though I very much doubt this, otherwise the dropout rate would be lower, once a week-ish) or it's a CATV problem, issues with power levels, or with your STB itself. If there's no pattern to the disconnects that is what I would favour. If they are always after a certain amount of time I would look at it being DHCP.
Again though so long as you haven't changed any configuration on your machine it's unlikely that your firewall will suddenly start to block the DHCP. Have to say I think the techy you were speaking to got it a bit wrong, and it's either your STB or CATV to blame