Cable Modem High Error Status

I have a Arris SB6190, with the following version: HW=3: SW =9.1.103AA65L. I have been intermittant very slow data downloads using Xfinity Speed Test. After resetting the modem (power-off) and  logging into the modems web view, there are no uncorrectable errors and the signal levels are between 4.9dBmv and 9.1dBmv and the download speed is as predicted (about 300-325mbps). After about a few hours, the speed deteriorates, there is about 35,000-72,000 uncorrfectable errors for each of 32 channels and the signal levels or some of the channels are -1.42dBmv. Is this a faulty modem or line or other?

Thanks