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The last week and a half or so I've been experiencing slow upload speeds (10-17 Mbps). I normally get 29-33 all day long, my plan is a 300/30. Feels like my upload speed is being throttled. My download speeds are roughly in the low to mid 200's (Docsis 3.0 modem) Doesn't matter what part of the day I've tried to run speed tests to verify it hasn't made a difference. Having my modem plugged directly into any of my computers via ethernet vs through a router doesn't make a difference either. Here is signal for my modem DPQ3212
DPQ3212 is not fast enough for 300/30Mbps. As per here, the DPQ3212 is only good up to 100/10Mbps. You need a modem with atleast a 24x8 tuner. You may be eligable for a free upgrade. What state do you live in? I think CA, AZ and VA offer the newer D3.1 phone modem. Otherwise you have to use two modems. The DPQ3212 for phone and another modem like the Netgear CM600 for internet.
With that said, do you notice if your upstream power levels changed when you upload speed went down? My guess is your on a upload channel or two that has congestion. Cox wouldn't throttle just upload if you were approaching your limit since you download so much more then you upload in general. However there is more download bandwidth then upload bandwidth, so you see congestion on the upload first.
Oops. Looks like the new D3.1 phone modem(eMTA) is only available to Gigablast customers, not Ultimate. Also, the three states are AZ, CA, and NV(LV). If you live in any of those three states, you might still be able to talk a employee at one of the stores to trade you one.