I think the power reduction is actually a symptom of the lower speed, not the cause. There's a maximum amount of power that can be transmitted on any one tone, so if the speed is lower, it's probably using fewer tones, and so the total power, from adding up the power on the used tones, ends up a lower number.
Have you noticed any noise on the phone line (do a quiet line test), or problems with phone calls disconnecting the broadband?
