I am having similar results on my line - it all started about 10 days ago. A couple of days ago, I noticed a BT announcement (I think on the status page) that said they were replacing/adding to existing FTTC and FTTP equipment at "trunk" level to improve "customer experience" ...... whatever that means. Maybe, now that take-up is higher, they are seeing contention at a higher level, especially if many have also signed up to TV too.
A more informed explanation may be along soon from one of our techies.
Colin