I have been having exactly the same problem with my Black Now TV box when watching shows from the Entertainment and Movies packs. I first noticed this on Sunday night with buffering every few minutes. When the stream eventually resumes, the picture quality is very degraded and unwatchable. This problem only occurs when watching streams from Now TV. Normal catch-up services (BBC iPlayer, ITV etc.) are unaffected by this problem.
There have been no physical changes to my hook-up, and like you, my internet speed is consistent at around 100Mbps. Also, my Now TV box is connected to my router via an Ethernet cable and the HDMI lead connecting the box to the TV is sound. This problem is clearly due to a fault with the Now TV service. I am particularly annoyed as I have recently purchased a 6 x Month Entertainment pass and a 1 x Month Movies pass, and both are now virtually un-watchable. Does anyone know if there is a software update planned to correct this issue ?
Best regards to all.