Crackly line
Another one to try is the BT line test. Plug a phone into your master socket (see previous posts) and dial 17070 (free). One of the options is a quiet line test. It's supposed to be quiet but, if you can hear noise/crackling this will seriously affect your upload/download speed because the S/N ratio is not good.
We had the same problem due to cr@p cable being installed by the housebuilders which then deteriorated over age (25 years). Once it was proved to be a cabinet to house problem (cos the noise was heard at the master socket) BT dug up the drive and replaced the cables.
Not a problem since!
Good luck...