If I look at my reports of how demand rises and falls, it certainly does seem to be a consistent pattern so I am not surprised. We don't generally see gentle slopes for increase or drop off in demand, it's usually a dramatic spike or dip, and it's usually so consistent you can set your watch by it.
Think about traffic patterns. How often can you tell almost to the minute when traffic will be horrible and when it will be light?
I am sorry, but the more I am given these explanations, the less convincing they become. Please consider ALL the evidence:
1) To use your own comparison, highway traffic is certainly not the same on a Sunday as it is on a weekday. Neither is internet traffic.
2) We are not talking of "almost to the minute" here. If I go to the trouble, I will literally be able to time this to the second. No traffic patterns (motor vehicle or data) are as precise as that.
3) Only NNTP and P2P are affected, everything else keeps flowing at a constant rate. How does congestion explain this?
4) Other ISP's apparently have their own dedicated "fast lanes", as they don't seem to be affected by the traffic.
Be that as it may, my downloads are now jumping from 0KB/s to full speed (1.2MB/s) and back -- something I've never seen in the morning. Must be some really dramatic stop/go traffic! My guess is somebody is fiddling with something right now ...