My house setup is there is a splitter on the outside of the house that provides cables upstairs and downstairs as well as the main level. The internet and tv is on the main level connected with a splitter on the inside of the house. The following is my modem with various splitters removed.
5) Have three different routers: WRT54GL running DD-WRT, RT-N16 running tomato, and WRT54GS2 running stock firmware. Have tested the internet with all three routers and with my computer directly connected to the modem.
6) Have attempted on 5 or 6 computer with various operating systems. Linux, mac, and windows all different firewalls settings ranging from none to some.
What HSI Tier you are subscribed to: We subscribe to Blast with 30mbps download and 4mbps upload. We are consistently getting 6-7mbps download and 1-1.5mbps upload speeds.
Re: [Speed] Pay for blast 30mbps/4mbps and am getting 6mbps/1mbp
Those splitters are making your upstream power level EXTREMELY high. As you can see, in the 3rd photo you lose most of your upstream channels because they all have too much of a power level. Without your splitters your downstream is getting near the high point and your upstream is right on the edge.
That's all I know(Or at least think I know :P). This will just be a bump for now until you get someone to help you fix this or give a solution.
I also wonder if you're provisioned for the 6/1 tier .