I've had this problem for ages now and had multiple attempts at fixing it, however they never worked. I decided to just leave it for a while, as I've seen that to be a suggestion, and see if the problem fixes itself. I assumed the issue wouldn't still be present and I feel like I'm wasting my time.
Your problem is Docsis and no good QoS for the upstream as you have many people in the area upload and the delay for you to send a packet buffers with everyone or maybe some people have their PC's infected doing DoS.
The problem got me thinking of another problem if no DoS was happening and that is VM could QoS after Docsis as a whole and that they try to limit and QoS upstream that matches four lots of 64 qam but if one person on them four channels has a 16 qam then VM would need to QoS as if everyone was on four 16 qam...
Thanks for your post, I am sorry you're having this issue with your broadband services recently.
I have had a look at your services on my side and can see there is an area outage, which is regarding an over utilisation issue like @Andrew-G mentioned.
This is due to be fixed - 30 SEP 2021 13:00 - but please be aware this is just an estimate, it could change depending on what the engineers find. However, I am sure they will be working incredibly hard to get this fixed as soon as possible for you.