You also didn't answer the question of whether the problem occured with out the Smoothwall router.
The best thing you can do is remove all your network stuff, & connect the modem direct to the PC & see if the problem re-occurs. If it doesn't at least you know the issue lies on your side, & if it does you know it's ntl's side.
You need to eliminate your own hardware/software before you start blaming ntl IMO.
[Edit]-I'm going to merge these 2 threads soon as we don't need 2 on the same subject.