Yes it is very frustrating - I've not even bothered to call tech support as they'll likely go down the scripted route of use the test socket, reboot the router etc etc. My line is very stable from monitoring DSL stats and BQM etc. and don't really want to disturb it.
Their network has gone a bit odd this time - when I had congestion previously it was peak only and generally slowed everything down. After a couple of months they upgraded something and overnight I got fast speed 24/7.
This time it is a bit different - different services, routes etc seem to be affected and affected services seem to be like it 24hrs a day. Speed tests are fairly consistent 24/day but single thread tests poor on some checkers. Downloads can be odd too, downloading the Windows 10 ISO the other day was exceptionally poor a few hundred Kbps, yet seconds later Windows update was pulling nearly the theoretical max for the line (~62Mbps). TBB download files vary (you've got to watch which browser you use as to whether its single or multi thread).
One thing to note on the Ookla speedtest at the moment the Vodafone Newbury server is unbelievably pathetic 24/7 (In another thread I think someone was told by customer services not to use it!!), yet other servers are much better, here are a couple of tests a few mins ago:
All single thread
tests, Voda Newbury:
Yet several other servers give like this:
For a while the TBB test gives me poor single thread 24/7 which always quickly falls in speed:
Youtube and BBC iPlayer seem relatively unaffected and can easily watch 4K.
I appreciate there are different routes traffic will take in the core network and different ones can be congested etc but this doesn't seem quite right.
Do I have a distant memory of there being a clause in the T&C about if they cannot provide satisfactory performance you should be able to end contract - although providing the evidence etc will be a pain, starting with the "sync" guarantee, fine if you have issues with sync, but then they should be sending an openreach engineer round!!