My upload speed is very bad, but download is fast. Happens on random days. What could be the problem?

I have the Netgear CM1000 modem and a Netgear Nighthawk Gaming Router XR500, using Coxs gigablast plan. My speeds are normally 600-800 down and 30-35 up, used to be a consistent 35 up. On random days, my upload speed will drop to 15-25 and cause issues when trying to stream games to twitch using my PC, playing on ps4. I've lowered my streaming bitrate to the lowest setting and nothing has helped. I've changed ethernet cables, power cycled my hardware, changed router settings, called customer support, they cant figure out the problem. Anybody having the same problem, or know how to fix my issue? Also have used the panoramic 3.1 modem/router combo and still had the same problem.