Not getting gigabit, stuck at 30mb/s, Low SNR on channel 159, issues with SB8200

Hi all,

I recently bought a new modem (SB8200) to fully utilize gigabit connection with my Alien Amplifi router.

I called Comcast to provision the new modem, which worked fine, after which I upgraded my speed from 175 mb/s to 1000 mb/s. Shortly after that, I received a jump in speed up to 700-900 mb/s, which was great. However, after power cycling a couple of times (suggested by support), once reconnected, I only received 30 mb/s. After many cycles and resets, I'm stuck at 30 mb/s. However, in the evening, my speed somehow jumped back to gigabit speed, and I cancelled my technician visit, as I thought it just took time to propogate. 

Cut to this morning, I'm back to 30 mb/s. Many cycles and resets later, still the same. 

Another issue I'm noticing is that a direct connection from modem to computer, I can't connect to the internet - only if I connect my router. I've tried multiple computers, but no WAN IP is being assigned.  This is a brand new modem. 

When I check my modem stats, this is what I'm seeing:

I know 19.0 dB is low for channel 159, which is the gigabit channel. Any reason for this/can I improve this? Or do I require a technician visit?

I'm going to try some more modem troubleshooting.

*EDIT*

I've successfully connected directly to the modem via ethernet after cycling while connected - still 30mb/s.  

Here's my most recent speedtest, wired to modem via cat6: https://www.speedtest.net/result/10102150568

At least I now know for sure that it isn't my router.

*EDIT 2*

Found a shorter coax, replaced it, and got high speeds again yay! But then, 20 minutes later, it slowly dropped to around 200, and now I'm back at 30 mb/s. Wonderful.