oddly my best suggestion is to stop rebooting! It takes time (10 days!) for the line speed to stabilise, and repeatedly restarting the modem can make the kit in the cabinet start to wind down the speed because the line appears intermittent. It's possible that your IP Profile (the speed throttle on the line, so they can manage throughout across the entire network) has dropped down a bit - if the line stays stable then it should lift again later. give it a few days.
If you want to know what the ip profile is on your line, the run the speed test at https://speedtest.btwholesale.com/ and do the additional checks. it'll tell you the ip profile for both download and upload - which is always slower than the sync speed.