Slow Connection

Why is my connection so slow?  It has been this way for months.  A tech came out in March (?) and changed the modem and worked on it.  I was able to get 90mbps for maybe a day.  I was told by a couple of people at Cox that it might be my modem (Arris).  I swapped it out.  I am still having the problem. 

At 2am or so I am getting 26mbps performance but I am paying for 100mbps.  I need this fixed - correctly.