This is a problem that someone i know is experiencing. Using optimum in connecticut he's paying for 100mb down and 35 up. Most of the time these speeds work, but around peak hours it will drop to under 2 mb up for short times, something like 5-10 minutes continuous, although the downstream will remain above 50mb. The second it returns to "normal" the upstream will return to something like 35 mb, and it can stay functional for hours until the next episode.
The company sent someone to the house and replaced all in home hardware and cables, couldn't fix it, and then washed their hands of the matter. Due to the short troubleshooting window and unpredictable nature it does make it hard to catch in the process. Regardless they should be doing something so i advised him to file a fcc complaint for now with all the relevant information.
It's above my head really but it really does interest me what could be causing such a strange issue. I don't know nearly enough about internet network infrastructure to form well educated ideas. The best i keep figuring is that something is overheating during peaks and taking care of itself, but wouldn't that effect the downstream as well?