CM8200 Cable Modem "Upgrade"

I am having the exact same problem.  I attempted to upgrade to the Gigabits package and have had the same experience as you.  I get about 2mbps on average now, but If I power cycle the modem. I can get 300mbps for about 10 mins then it is back to 2.  I have had two technicians come out both said there was nothing wrong with my end.  I exchanged the modem but the issue continues.  I have worked with support daily, and today finally was able to speak with tier2 support.  I have a third tech scheduled for this Thursday.   

What I learned.  docsis3.1 modems operate on  different frequencies than docsis3.0, and if there are not a lot of users on 3.1 it could likely that not many people are having issues because users using docsis3.0 are not seeing any issues.  Making it more difficult for them to troubleshoot and why the tech's are not finding anything when they show up.  Being that docsis3.1 is new to my area "NE Mesa" I am probably the only one having an issue in my neighborhood if everyone else is still utilizing the docsis3.0 modems.  This explains why my upload speeds have not been affected at all, and are normal. 

I truly feel my issue is related to a low/misconfigured signal specifically for docsis3.1 upstream from my neighborhood, but getting the right support area has been challenging. 

Update:  After the call yesterday with Tier 2 support I feel like I have found the right path to getting this resolved.  Hopefully after my appt on Thursday I will know more.