well NTL do 10meg connection...

For that i'd insist on a router.
the main reason being... the max that a coaxial cable can manage at any one time is 10 meg... so with your tv coming down on different frequencies as well and having to deal with demodulating two different tyoes of signals... it's probably getting over heated or stressed about it some way.

Also... Try a firmware update on the linksys... Ocassionally mine gives me grief but not constantly.

If they still say you can't have a router tell them you want to cancel your subscription because you're not getting the service you paid for