CM1100 Slow Speeds & Frequently Disconnects With Several Uncorrectable Codewords

After looking at your log, I'd bet you're correct in it being a line issue. How much does mediacom charge for rental? If they'll gaurantee that you'll get gigabit with their equipment (and you don't have to sign a log rental contract) rent their device for a month and make them their equipment/signal functioning well enough to fully run gigabit speeds. Once they get the line issue fixed, return it and run your cm1100. Might cost you $15-$20 for rental but it lights a fire to get them to fix it probably worth it versus numerous visits and slow speeds.