Should I be concerned of my downstream going to -8 dBmV?
From http://forums.timewarnercable.com/t5/Connectivity/FYI-basics-of-signal-power-level/td-p/116692 guide, -8 is borderline. I know temperatures can also affect this since it is hot with the summer. It was mid 90F degrees outside and almost 90F degrees in my room where the hot cable modem is. Attic and walls would be much hotter where the coax cable lines are hidden.
There was one recent short outage on Sunday when the cable Internet connection went down, and the cable modem decided to autoreboot by itself. DVR's cable digital TV was still working though when I checked during this outage. Logs of that short outage.
Cable modem's logs and statistics can be found in https://paste2.org/h0mzPa39 (couldn't copy and paste into here due to the forum's 20K characters limit -- boo!).