I feel your pain.
I have the same problem. Line speed drops to stupidly low speeds, sometimes as low as 2Mbps.
Rebooting the router restores a reasonable speed for a short while before it drops back down again.
I have been rebooting my router two or three times a day for the last few weeks.
Their helpdesk is useless.
I am very close to taking my case to OFCOM. I can only suggest that enough unhappy customers do the same, then perhaps sufficient pressure will be put on NOWTV to do something about it.
They clearly don't care about their customers.