With your signal strength, you must have a wiring issue.
For DOCSIS to be 'possible' on a multi wire dwelling, DOCSIS specifies a minimum of -8dbmv signal strength. ( after all home wiring, measured at the modem, but you have no splitters so its like a straight line )
With that RG11 drop cable I see in your image, resistance is pretty minimal.
With no splitters, you should have either perfect signal ( with minimal at the security box being +10 ) or way too much ( blasting ) at the modem.
noise levels looked okay at the modem, however it is not nearly as accurate as a CATV meter.
There's is just no way you are losing 20-35db through healthy cable/fittings.
At the downstream you'll lose about 5db per 100 feet through RG6, closer to 3.5 on RG11.
Here's what I think the issue may be, Bad directional coupler in the tap, or a faulty line extender before the tap.
If there does happen to be alot of noise on your wiring, your modem sucks at detecting it.
It could also be conductor suckout, but that is usually more detrimental to your upstream traffic.
Edited by Xinoxide - 10/21/12 at 6:53pm
In a nutshell. I think it originates on the carriers lines.
If not, I think you may have a bad cable, possibly a kink, or a bad fitting, possibly because of rot or suckout. See if you can get a tech to run a new line straight to the modem ( looks like it runs outside along the house )
Just make sure that tech gets up there and checks that TAP one way or another. It should be required by his superiors to certify your premise as well as their line equipment before he leaves. That is mandated by the FCC.