I've been getting exactly the same (I'm on the Banbury exchange).
Speed tests (no matter which speed test site I use) struggle to get above my old 4.5Mbps BE connection, yet as soon as we get outside of peak hours the speedtests are a rock-solid 37Mbps.
From August last year, through to a week or two ago, I used to get speed test results of 37Mbps irrespective of the time and day.
Going by many years experience (and regulars of these forums will have seen it many times before with the old ADSL products), there's little doubt in my mind it's either localised congestion (can't be national because the forums would be full of complaints) or BT are throttling web traffic on their Infinity product.
Or BT have a "bad boys" list and are throttling heavy (ish) downloaders.
My issue is definately not with my connection (sync'd at maximum speed and it only ever drops sync when I reboot the modem - it's easy to spot this with a large ISP like BT as your IP will most likely change when your modem re-syncs - between October last year and the begining of March this year, I had the same IP address).
If I set up a Usenet download it will go at maximum speed at any time of the day (even when speed testers give results of around 4Mbps for my 40Mbps connection).
Therefore it's unlikely to be congestion (if the networks - locally or nationally - were congested I wouldn't be getting full speed from Usenet).
Oddly enough it all started around the same time as BT started making 80Mbps connections available to new and existing customers (i.e. came out of any trial phase, and went into roll-out mode).
My money is on throttling.
EDIT: pings are rubbish too - just got 26ms to a server in Coventry (about 30 miles up the road and the one Speedtest/net selected as my closest server).
I normally get pings at (or just below) 10ms.
vDSL2 FTTC Infinity with BT
DL Sync 40Mbps
UL Sync 10Mbps
Edited by adebov (Sun 22-Apr-12 20:40:58)