If when the speed drops the line remains in sync at the same level as before then the issue is not with the copper line.
It's either in your home, or on Talktalks part of the network.
Are your speed tests being done over wireless or wired?
If wireless you must try a wired test to rule out the wireless signal.
If they are wired it's more than likely a Talktalk issue, and I wish you good luck getting in touch with someone technical that can help you.