Speed issue, modem log shows high number of uncorrected errors

I currently have the Gigabit plan, but am only able to achieve around 500-600mbps. Didn't really notice as I have been steadily upgrading over the past few years from slower speeds (200?) up and up until I landed here, mainly for the improved upload speeds. I mostly use wireless where I can usually get 200-350 or so, which is plenty, so I just now realize that I'm not getting the full speed on the down side.

I use my own equipment and have a Motorola MB8600 modem which is gigabit capable. I did test straight from the modem via CAT6, bypassing the router altogether to confirm that's not the issue. 

Checking the modem logs I see a good amount of uncorrected errors on the OFDM PLC channel (about 10,000 in a day), and that one is at the lower limit for signal (-8.0). All other channels have better signal, SNR within range, and uncorrected errors in the double digits so not as bad but maybe still indicative of a problem. 

Again, down speeds end up slightly below 500mbps straight from the modem. Up speeds are 23mbps which I think is correct for my tier (plus overprovisioning). Any ideas as to what this might be or how to fix it?