Spark modem HG659b

Prime video.

Hello. i'm streaming Prime video via my above modem and Google Chromecast. 

Using speedtest.net my download speed is 61.11 mbps WHILE watching a programme on Prime Video (its 5:30 pm on wednesday).

I'm getting periodic interruptions to the program when it 'freezes' for 1/2 a second & then resumes. but 3-5 minutes altewr its another small interuption. Not much, but frequent enough to be annoying. 

I'm wondering, if its possible to identify the cause of the interruptions ?