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!).
I doubt that you have a signal level problem attributable to the temperature in your attic. I would be more concerned about the several RCS Partial Service messages in the event log. These indicate that your data modem is unable to receive information on one or more of the DS channnels assigned to it for internet data communications. It's time to call Spectrum and arrange a home tech service visit. The tech will attempt to determine why those channels are dropping out at your location.
HINT: Swapping out the modem or replacing the drop cable will NOT fix this problem. Don't let the service scheduler or home service technician tell you otherwise! Do NOT let them reset your modem remotely and then cancel your appointment because "your signal levels are back to normal again."
Chances are that other Spectrum internet subscribers nearby are having similar issues, since the DS channels are shared among all of the cable internet customers in your area. It takes three users calling in to Spectrum with the same complaint to start a field trouble investigation, so if you try to wait it out or fix it yourself, you are only delaying a restoration of normal connections by the maintenance teams.