Unlike fixed speed services, 8 Meg adapts to your line conditions so if there is excess noise on your line e.g. caused by interference, your noise margin will be reduced and your sync speed may drop as a result. This could happen if your router is plugged into a different socket in your house.
It's advisable to keep the router switched on if possible.
Which router do you have? Can you post the line stats i.e. current sync speed, downstream attenution and downstream noise margin.