I have a similar problem. No issues for the first couple of months of service. Now the download speed is ridiculously unstable. We are meant to get 36mb/s and it might reach that for 15 seconds but then rapidly drops to 2mb/s or 700kb/s... then goes up to 8mb/s and on in a cycle.
I’ve tried altering distance to the router, used a different router as an access point etc. No change.
I’m thinking the phone line is rubbish. At our old address we had Virgin fibre and the speeds were very very consistent. We had a router issue at one point but new router and Virgin Media Hub as a modem instead and the issue was resolved.
I'm going to badger NowTV to check the actual phone line because different equipment doing the same thing on multiple devices means it has got to be a line issue.