Ok so I had the same issue and I upgraded my modem to SB6141 however I was still getting low speeds. My modem kept rebooting and rebooting once they added my new modem online. It was doing this for 3 days. My unit was not bonding all 8 channels. I scheduled a tech to come out.
Come to find out I had a Filter on the main line coming in since I dont have Video with them. The Filter went bad. When he checked my connection from inside it said I did not have a filter. However when he climbed the pole he saw the filter and did readings up on the pole. He removed the filter and IT WORKS PERFECTLY FINE.
Have them check the filter from the pole going to your place.