Noise/interference on cable system affecting speeds

I recently upgraded to the 600/15 tier with a D3.1 modem, and I never get anywhere close to the advertised 600 Mbps. I've even tried bypassing all my networking gear to rule out any issues, going directly from a windows PC to the modem over 1gig ethernet, and the highest I'll ever hit is ~390 Mbps.

 

Upon further investigation of the modem logs, there is a large number of uncorrectables on all 32 channels. What's funny, though, is that the uncorrectables show up in large numbers randomly; they don't build up consistently over time. For example, I'll reset the FEC counter in the morning, go the whole day without any uncorrectables, then all of a sudden, get a million across all channels in a span of a few minutes late at night.

 

This leads me to believe there is some device on the cable system that is introducing noise or interference when it's being turned on. It just doesn't make any sense that there should be such a high number of uncorrectables in spurts.

 

Screenshot of modem levels for reference: