Connection issues with new SB6141

Hey, y'all. I've been having a few issues with my SB6141 since installing it last Friday and was hoping I might find some help here. I've got Cox Preferred internet and am located in Baton Rouge.

I bought a new Arris/Motorola SB6141 from Amazon and installed it on Friday, running it through my router (Netgear WNR1000). After the install, I was automatically redirected to Cox's website, where everything automatically authenticated. Things worked fine for about an hour until I noticed a significant drop in download speed. Several modem/router/computer power cycles later, I found myself still having the same problem. Using SpeedTest.net, I would find that my "good" connections would average between 30-35 Mbps per download. After anywhere from 5 minutes to an hour or so from my last power cycle, I'd lapse into a bad connection and average from 0-2 Mbps per download.

I did some preliminary research and determined that my router had shipped with out-of-date firmware - 1.0.6.12.  Since I had to go out of town for the weekend, I figured I'd let the modem sit and see if Cox pushed new firmware to it. When I got back to town last night, I noticed that newer firmware (1.0.6.14) had been pushed, but I'm still having the same issue. I can get a good signal for anywhere between 5 minutes and (last night) 2 hours, but I inevitably lose connection speed.  I tried calling Cox's automated service, but ultimately ended up with the same results.

After some frustration, I spoke with a live tech last night. She walked me through another power cycle, but I ended up with the same problem while we were still on the phone. We then bypassed my router and connected directly from the modem to my computer. After another power cycle with that setup, I actually got SpeedTest results from 50-60 Mbps, well above anything I'd gotten previously. Still, after a little bit of time, my connection speed throttled again. I reconnected everything through my router, made sure its firmware was updated, and ran another test. I ended up with results between 40-50 Mbps, but again, the good connection was only temporary.  Frustrated with everything, I shut down my computer and unplugged my router and modem for the night.  I tried both setups this morning and ended up with similar results. 

If any of y'all have any insight on what might be going on, I'd appreciate it.  Below are some screenshots I took of my modem's signal and log pages. Hopefully they might be of some help.

These first two are from last night, with the modem-router setup.

These are from this morning, with the direct modem-computer setup.

I've obviously noticed the one channel that repeatedly gets a high number of uncorrectable codewords, but I've got no idea what I'm supposed to do about that.

Thanks for taking the time to read this lengthy post. Hopefully a Cox tech might stumble upon this post and be able to offer some assistance.

tl;dr: Installed a new modem; intermittently drop connection, requiring a (or several) power cycle(s) to get everything back to top speed; lose connection again. Happens on both a modem-router and modem-computer setup.