Your downstream and upstream signal power and Signal to Noise ratios look fine. Given that fact, and from what you are saying regarding your tv signal as well, I would say that you have a cable connection problem, or perhaps the cable itself is failing. You should call into tech support, indicate to them that you are experiencing dropouts on both the computer and tv and request a visit from a field tech to inspect the cable that runs from the external connection point all the way to the 2 or 3 way splitter that you have in your basement. The only common component for both systems is the cable and the splitter. If necessary, put tech support on speed dial and call in everytime you experience a dropout. The cables don't last forever, and it doesn't seem to matter if its above ground or underground. Every once in a while they need replacing. As they fail however, they tend to cause a lot of frustration. Take a look at this thread, which also had issues caused by a failing cable:
In that case the power levels were off and there was only one upstream channel instead of three. The tech visit appears to have solved much of the problem.
Where is the CGN3 located? From your drawing I assume that it is upstairs, with an RG6 cable feed from downstairs. If you look at the sides of larger white cable, downstairs and upstairs, it should have RG6 stamped on it, along with other cable data. Can you confirm that RG6 is the cable type? Just want to be sure about it. Also, if you look at your ethernet cables that run to each computer, you will see Cat 5, Cat 5E, or perhaps Cat 6 stamped on the sides of the cables. Can you have a look and confirm each cable type.
Lastly, on the back of the CGN3 you will see the connection LEDs which in this case are green for 10/100 Mb/s and Yellow for gigabit connections. These indicate the interconnection rates between the CGN3 and the device that it is connected to. If the LED is green, that means that whatever is at the other end of that connection will interconnect at a maximum of 100 Mb/s. If it is yellow, then the interconnect rate is 1 Gb/s. There is a problem with the CGN3 in that it thottles back the download rate when its connected to a 10/100 Mb/s device. Looking back at your posts, it appears that the pc that your results show is running at 1 Gb/s, so depending on your service plan, you should be seeing max download rates above 200 Mb/s if your on a 150 Mb/s plan or above 300 mb/s if you are on a 250 Mb/s plan. I know that your realtek PCIe GBE Family Controller Properties are set for 1Gb/s but can you have a look at the indicator LED to confirm that its running at 1 Gb/s. You can also check on the pc itself by right clicking on the internet connect symbol on the lower right hand corner of the monitor, select Open Network and Sharing Center and then select the local Area Connection on the right hand side. The status panel popup should indicate a 1 Gb/s interconnect with the CGN3.
Ok, so, have a conversation with tech support and confirm the cable types and we'll go from there......