24 years as programmer so I do understand they are having issues that are seemingly unrelated.
G.INP sounds like little to no communication by BT.
But the rest has to be down to plusnet whether it be internal load balancing, peering/routing agreements or not enough bandwidth etc.
But as a consumer should I have to be concerned with this? It should, barring line faults "just work".
plusnet Unlimited Fibre (FTTC). Sync ~ 63.83/15.7Mbps