New Cox 100mbps service seeing speeds of 4mbps to 120mbps depending on time of day

Recently switched to COX Premier Internet only from DSL.  

My euipment:

Apple IMac

Apple Airport Extreme Basestation (router)

SB6141 (DOCSIS 3.0)

Upon installation, I am seeing speeds immediately of over 100mbps.  Very happy.  That was around 4pm yesterday.  Went through several power cycles and reboots to make sure everything was configured right.  Also did a factory reset on the Airport.  Everything looked good.  Then around 6pm it dropped to about 60mbps and then around 7pm - midnight is was getting anywhere from 10mpbs to 3mbps while the upload stayed around 15mbps!  

I understand the whole DSL vs CABLE and network traffic, peak times, etc.  But is this what I can expect during peak use hours?  Serious less than 10% of the advertised speed?  

I made no chances to the configuration and got up at 4 AM to observe speeds.  They returned to about 100mbps, so I don't think there was any other variables besides time.  Please share your experiences.  Thanks!!