Amazon Prime LG App Bitrate

[Biker 1]

I have a 43" LG smart TV, couple of years old, with built-in Amazon app. It also has BBC iPlayer & Now TV, both of which stream 1080 HD with no buffering/loss of quality. TV is connected via ethernet to a BT Hub 5, with 70 meg broadband.
I have signed up to 30 day Amazon trial. It seems that the first couple of minutes of any programme are in SD or worse, & then the app settles down in HD. However, any fast/action sequences suddenly drop to SD or worse, & other times the quality goes for no apparent reason. Is this an LG specific problem? Would a Fire Stick solve the issue??