I am experiencing similar problems the past week, averaing 20 to 35 MB/s for a service that sohould be in teh range of 175 to 250 my normal performance in the past several years. Something has changed in the network, IMHO. I also went through multitudes of reboots, powercycles, and even a equipment swap for the same model Arris 1682G as was the old unit, after bringing it home and going through configurations and a couple of reboots. Still getting the same performance in the 20 to 35 range.
Looking forward to the Technicians visit on Friday afternoon to investigate this issue further, where I suspect a failing overhead cable run from the pole to the house, which was stressed when a tree limb came down on it last October.