The graph shows a total disconnection overnight, do you switch your router off at all? This won't be why your connection is dropping, but it's worth noting that keeping your router on as often as possible will give you better speeds, generally.
The more your connection drops/gets turned off, the more likely the automated line management system is likely to kick in and try to slow your broadband down, it does this because it can't tell the difference between a drop in your connection, or you turning it off, so it simply slows your connection down in a bid to try and stabilise the line.
Regarding the dropping connection, is your router located near anything electrical at all? I'm wondering if your router is suffering from any electrical interference (more common than you think!). Is it near a TV, Radio, Boiler or anything at all? Have you had any outdoor works in your locality that might have caused a disturbance to your line?
It could be one of so many different things, it's really useful to try and narrow down what the cause could be. It could be a fault outside of your control, which, if is the case, we can get that fixed for you.
To be sure though, if you haven't already, it's always worth connection your router into your Test Socket and if you've got a spare Microfilter, try that as well and see if that helps stabilise your line. Just want to try and help ensure it's not something being caused by faulty equipment (it does happen) or anything internal.
It's frustrating when your connection keeps dropping, so anything that can be done to try and rule out anything internal causing the problem is really essential and will help speed up the diagnosis of the problem.
