I have a similar problem. I have been on BT Infinity Option 2 for 2+ years completely trouble free. After recently experiencing phone line noise problems my broadband speed reduced dramatically and my router was reseting every 5 minutes. The speed reduction was obviously due to instability with the phone line. The line was repaired but the broadband speed did not return to normal. Eventually BT sent an engineer to reset the DSLAM port from my network point in the house. The tested speeds by him were 67Mbps download and 20Mbps upload. I ran speedtest.net with him and got 63/18 with 20mS ping latency to Manchester. A couple of days later - still with a stable line and no router resets - I repeated the test and got 54/4Mbps with a ping latency of 32mS to Manchester as before. I then did a test to a server in Miami 4350 miles away getting a ping latency of 141mS a download speed of 42Mbps and an upload speed of 4Mbps. That says to me that the upload speed is capped but BT Tech Support will hear nothing of it. They are especially cute in saying that upload speeds are not subject to any form of guarantee. I still have the limited upload speed. By the way, before phone line problems I had an upload speed of around 12Mbps.
Following some research on the web I switched the WAN cable to the Gigabit WAN interface on my router and did a full reset. There was no change at all until just after 48hrs steady running. The router then reset itself. I did some checks and I now have 58/11Mbps to both Manchester and London and 38/9Mbps to Miami. So looks like I was wrong and that the system will eventually heal itself.