slow internet at night

its been weeks now that ive been having this issue. im suppose to be getting 50 or more mbps but actual speed goes down as low as 5mbps at night even directly connected to my modem. im at las vegas area and i need help. already called cox tech in 3 diff occasions. had my modem reset, connect pc direct to modem and still getting the same speed. 1 tech told me they are having an outage around my area? i dont know what that means.. another tech just reset my modem, and then 1 tech checked my modem signal from his end and he said modem signal is pretty good. anyhow, internet slows down pretty bad at night goes back up at advertise speed on day time.