I work as Tech Support for a Broadband provider (not plusnet) and this is something we see regularly.
It is most likely your line, I would expect plusnet to have already covered this with you but if not;
Connect your micro filter and then router to your telephone socket test port, most people have one hiding behind the bottom section of the master socket face plate. Taking off the bottom section will disconnect any internal wiring from your line and give you the best connection possible.
Track when you disconnect, is it always at the same time of day? If yes what's going on then? Think of anything and everything you can from heavy traffic on the road outside during school runs to when the microwave is in use inside? Does it get better/ worse depending on weather conditions? Is your phone line ever noisy or quiet - even just occasionally? Answers to these will help determine REIN or other causes of intermittent faults
Once you have this info speak to plusnet again and see what they can do next.
Don't upgrade to fibre expecting a miracle fix, most fibre installations are from the exchange to your cabinet and then it's the same copper wires from the cabinet to your home. If there is a fault in the cable it is most likely to be in the bit between you and the cabinet and if it is REIN then you are unlikely to fix it by changing to fibre.
An occasional drop can be expected, every day is a fault BUT is unlikely to get fixed until it gets worse as is very difficult for the Openreach engineer to track down as they really need to be on site with their diagnostic tools plugged in when it happens.
If it is REIN there is no guarantee of fix if the cause is something out with your control. The REIN engineer may find it is something in a neighbours house but if the neighbour declines to do anything the engineers can't make them 