is your wired speed also bad ( that rules out wifi interference ) , if so, you need VM to look at total load on the local street box when your neighbour is busy uploading/streaming/ torrenting./ running his own youtube channel.. whatever he gets up to
yes I did, had my laptop plugged in directly to the virgin router as modem but also did a reset before to use it as a router and no change. If I turn it off and on, the speed is good but if I stop using for an hour the Internet is really slow. I have to be honest and I wonder is the speed is shared and if only the new IP gives me the speed I pay for, if I do not use and the neighbour is pushing for more he gets it... The router I am using is a Linksys WRT1200AC so is even better router then the built one.
Anyway, I tried all combinations possible and no change. One thing that always sorted the turn it off and on but many times I am upstairs, even in bed and need to come down to turn the device off and on and back to normal.
The next time you notice the speed drop can you post your router information please. To do this type in 192.168.0.1 in the address bar, no need to sign in, on the top right you will see router status. Within here you will have downstream, upstream and network log. You can copy and paste this information, that would be great.
Also it will help us further if you can set up a Broadband Quality Monitor. If you do set up the BQM make sure that you go into your router settings > Advanced Settings > Ping > and tick Respond to ICMP echo requests sent to WAN IP.
are you physically linked to the neighbours install i.e. has an engineer tapped one cable to add another ? - take a look at outside the houses and trace the incoming-from-street cables, post photos if you think you are linked. That is sometimes done as a temporary fix so if the engineer then "forgets" to come back & do it properly...