Make sure it isn’t disconnecting intermittently (or being turned off at all). BT’s equipment at the exchange is designed to force your speed down if it detects problems (in an attempt to stabilise the line), and constant disconnections either through fault or turning off the equipment, even once or twice a day, are a symptom it recognises.
We had a similar problem; they sent out a BT engineer who found out the wire connection to the house (up the ladder!) was faulty. He tightened it up and phoned the exchange to reset the equipment. This was free of charge after a phonecall to Sky.
However my Aunt used to turn off her modem/router whenever she wasn’t using the internet; unsuprisingly as a result, she had unbearably slow internet!