Internet speed drops from 17+ Mbps to less than 1 Mbps (about 100-600 Kbps) EVERY night between the hours of 9 p.m. and 6:30 a.m. (speeds confirmed by Testmy.net)

My Problem:  Every night starting at 9:00-10:00 p.m. my download speed drops from its normal 15-20 Mbps down to a crippling 100-600 Kbps.  You can see my testmy.net (Cox's recommended speed test) here: http://testmy.net/stats/?&t=u&d=11032012&x=1&l=50&q=dvenable  I also have an image posted below  of speed test results from another night.

Is anyone else having this problem or has had this problem in the past?  I've contacted customer support several times, but so far it has not been fixed.  It has been like this for months (yes, months) and has not been fixed.  The only thing that has changed is that it went from doing it some nights to consistently failing every night starting at the 9 o'clock hour and continuing to fail throughout the night.

This is not isolated to my house either.  My father (who lives on the same road as me) and my friend (who also lives on my same road) is having the exact same problem at the exact same times.  This is incredibly frustrating.  I am essentially unable to do anything when the bandwith chokes after 9:00 p.m.   There have been two Cox guys come to my house (and besides the fact that they said a cable came out of the pole by just pulling on it) who said the problem is not an "in house" problem.  

Sometimes the internet even bounces back and forth from 15Mbps to 200-300 Kbps.

My wife and I have been dealing with this for months and if we can't get reliable service we're going to have no choice but to switch to the slower, but albeit more reliable (at least between the hours of 9:00 p.m. - 6.a.m.) DSL.  They offer a max of only 6 Mbps connection speeds, but when you're getting speeds slower than dial-up for up to 8 hours a day 6 Mbps would be an improvement.