Is this occurring at all time throughout the day or just during evening hours? Also, if using a router have you reset/rebooted it?
Most speed loss can be attributed to signal related issues. If there is a splitter between the modem and the wall outlet you can bypass the splitter to see if it continues to be slow or even try another cable outlet if that is possible. If still no change PM the phone# or account# and I will check things over on my side.