Unless you've got an attenuator (sp) on the back of your modem there's little you can do about power levels, not sure if NTL will send an engineer out untill you start to have problems but take it from me xmas is not a good time to have an NTL service go west (they gave my phone line to someone else last year then tried to charge me for all the sudden international calls!
)
If it does have issues and you do have a little extra plug or two on the coax cable connecting to it you can try taking one off, they are normally colour coded with a number (e.g. -2db, -3db). I've had to do this before when levels droped, it's the first thing an engineer does anyway, just check the levels afterwards to make sure they are okay and do it by the smallest amounts first.
If you have non then it'll need an engineer to boost the power either from the box outside or the one down the road.
There's a sticky topic in these forums with expected values, I need to check mine soon as it's been a while.
PS This is at your own risk cos I ain't no expert or nuffin or like from NTL or stuff.
