NowTV have identified a bug in the current firmware on the nowtv standard black boxes and are looking to push out a new firmware ver 8.0.3 in the next two weeks according to this linked thread below.
I've been having the exact same problems, I've tried finding I fo all over about it.
I have recently bought a wifi range extender to make my talktalk tv box wiresless, so I decided to connect to that rather than directly to the router........hallelujah!!!!! It worked.
It seemed to change the wireless channel from 36 to 9.
I think the router is having problems connecting directly to the now tv box.
Since the latest firmware was pushed out to the nowtv smart boxes a number of TalkTalk customers have reported various problems with their smart boxes or having difficulty streaming the NowTV content via their TalkTalk Routers.
Here's some threads linked below.
Maybe try the suggestions in these linked threads above, where some TalkTalk owners had success, where others have reported no joy.