Over this past summer my wife (who is a teacher and was home for the summer) started complaining that our Internet would go down around noon every day. She said it would usually be out for an hour then come back for a few minutes then go back down again. Usually by the time I got home from work in the evening it was fine so I never bothered calling Comcast support. This went on for a couple months until finally I was home one Saturday and noticed the problem. Initially when my wife reported this to me I figured maybe they were performing maintenance but surely any maintenance couldn't be going on this long.
So I got on the phone with Comcast Support and they performed some diagnostics over the phone and determined that I did have an extremely weak signal and they would have to dispatch a technician as they suspected the issue was with the line. A couple days later the tech came out while I was at work. My wife was told that we had water in our cable line and it would need to be replaced. Now, I am not sure how water got in the cable line in the first place as we had the driest summer in years in this area. Nonetheless it fixed my issue.
I assume what was happening was in the morning the water in the line was cool and didn't affect the signal. As the sun rose through the morning the water in the line would get hot and degraded the signal. Then as the sun set in the evening the water would cool down and my signal would come back. Not sure how common this is but I just thought it was kind of interesting.