What ping times do you get from www.bbc.co.uk ?
Could it be packet loss on Gfast line, or some sort of wired ethernet issue between the MT992 and the HH5a ethernet wan port?
eg. faulty ethernet port on HH5a ?
Do you have a TalkTalk router to connect to the MT992 ?
Try this Windows app for pinging external websites to detect packet loss.
https://sourceforge.net/projects/winmtr/
Provided your IP address doesn't change frequently, this may be useful for detecting packet loss too:
https://www.thinkbroadband.com/broadband/monitoring/quality
(Remember to enable ping on hh5a wan port)