why can't Cox fix our internet issues

This has been going on for months now - our internet slows down drastically or won't even connect between the hours of 8:30 and midnight.  We have replaced the modem, the router (2x), etc. - this is whether there is one person or four people trying to use it.  Cox keeps stating they show it's fine.  However, when I use my WiFi (or one of my kids use their's, we connect immediately and have no issues.  Cox was supposedly going to monitor it for 30 days, but I never heard anything.  Also, our bill keeps increasing and the service  (internet and cable tv) keeps getting worse.