I'll check the upload speed when the slowdown occurs. My tool currently doesn't support automatic upload tests, so I'll need to do it manually. Is there a reason you're curious about the upload speed?
A technician came out today and checked my line quality. He found that there was a slight signal issue at the connection point to my building, and also replaced a water damaged cable which previously fed my apartment, but he found nothing that would account for the speed pattern I am seeing. He agreed it is likely a network issue and thought that TWC might be doing work on the system during the time when I am experiencing the slowdown. He was not able to provide any evidence for this though, and claimed that technical support (even tier 3) would not know about such work either. This seems a little dubious to me, of course -- as if they'd have employees out doing work every night, slowing connections to a crawl, without notifying anyone else in the company who would be receiving complaints from customers?
The next step will be for TWC to run additional tests to try to pinpoint the issue. I am going to keep running my speed tests in the meantime.