Slow upload overtime - Time Warner Internet

This question is about "Slow upload overtime", with Time Warner Cable internet and apps. My upload getting slower after awhile. It will be at 35Mbits for like 2 hours then went to 20Mbit and could not get back to 35Mbit unless I reboot modem. Endless cycles. Also I only have internet and no TV, why do I need a splitter? This is on 1Gb package.

Slow upload overtime?