We had very similar symptoms to those you've outlined.
I've just had a conversation with PN customer services following my post in the forum and raising a support ticket (as they requested I do). Essentially Plusnet have quoted for - and in fact provisioned - a service they will "never in a million years" be able to deliver.
The reason for this is our distance from the cabinet. I suspected ~1.5miles, 'Nick' (PN bod) confirmed it's actually just over 2 miles. Consequently our line is having to work incredibly hard to try and hit the 80/20 profile we're apparently paying for. This in turn leads to massive performance hits predominantly due to the low upload capability.
It's worth trying to find out which cabinet you're connected to and see if you can identify it in your local area (a ball-ache I know). We're now going to go down to 40/2 to see if that helps the situation and if that fails they "won't charge" for a further downgrade to ADSL. It seems bonkers, but this would in theory give us a faster, more stable connection - that is certainly our experience (old ADSL with BT was not amazing but it was what it was, this is just unusable).
As I said in my thread - it isn't commercially viable for ISPs to provide a good network for rural communities. So they don't.
