Hitachi 48" tv, the white NowTV box
Given that you are using the 'white' NOW TV box means that you are connecting to the internet via WiFi.
Personally, I'm not a fan of WiFi, so I'm unable to offer any help!
I'm as confused as @SeeMoreDigital with relation to subtitles as this is not a service we have at the moment on NOW TV.
With regards to your picture quality, most likely reason would be the speed of your Internet connection.
How far away is your NOW TV Box from your router ?
Any other cordless/wireless devices near the router or NOW TV Box that could create interface ?
The most common reason that your picture would deteriorate is due to the Internet speed.
Can you try a reset of your router and your NOW TV Box to see if this makes any difference ?
Also if you are with BT for your Internet service could you disable Smart SetUp from within your router settings as this has an impact on some devices.