You have what sounds like a classic case of an upstream impairment that is time-dependent. Do you by chance have any heavy equipment around you that fires up at 8:40pm? If this is a new occurrence, see if there is a streetlight that turns on at this same time. Something is likely coming on which is emitting a lot of ingress into the upstream. It is preventing your RNG-REQ from reaching the CMTS, hence your T3 Timeouts. In addition it is causing packet loss, which you are seeing as latency, because in a TCP/IP flow, the packets are just being retransmitted. Of course I am presuming your are basing the latency off of HTTP page requests or gaming results. If using VoIP or video conferencing, which is UDP-based, there is no packet re-transmission and so you would have noticeable packet loss and hence degraded service quality.

I think it is important you work with your cable operator and have them measure errored code words (measured on the CMTS) on your modem during the evening hours (8:40pm and after). This will be the only way that they will actually see the problems you are experiencing. If they test in the day time, everything will look just fine. Ideally, you may be able to identify the source of ingress and point it out to the cable operator so that they can fix the leak in their plant. This is often a difficult battle for the cable operator and everyone pays the price.

Good luck, let me know how it turns out.

-Brady