This indicates a problem on your upstream connection. Confirm that all connections are tight. Replace interior cables with brand new high quality RG6 if practical. Remove all unnecessary splitters. If problems persist, contact Cox and request a tech visit as T3 timeouts are often caused by problems outside the boundaries of your property. Post your modem signals and full logs for more complete advice.
Uh, not sure why you're downvoting when I'm trying to help. That's technically true bit it makes it sound like there's a problem with the Cox CCAP. If your modem sends several RNG-REQ messages without receiving an RNG-RSP message it logs a T3 timeout. If the whole CCAP was down you'd see a complete loss of connectivity. The reason the modem isn't getting a RNG-RSP is because the RNG-REQ is never getting to the CCAP. This is almost always due to noise on the upstream which only a Cox technician can fix.
There is a fundamental flaw with the cable company dogma that we see repeated here.
In my case, and in others reported here, the events are clustered in time. There is no connection flaw mechanism that would lead to time-related drop-outs. However, a problem related to congestion and problems at the upstream end in dealing with congestion would explain time dependence.
The problem with having a Cox technician examine the problem at the customer end is that he will never find the problem if it is some sort of congestion issue.
I believe that Cox knows exactly what the problem is, and is not telling us because they do not want to spend the time and money to fix it.
But maybe I am wrong and Dave's explanation that these problems are all unrelated coincidences having nothing to do with each other is the answer.