I am seeing a similar issue here. Slowdowns for me are usually at night as well (upload speed only though), so no technician will be there at that time, and no one at VZ phone support will do anything to help besides send a technician to your house, so nothing will be done to fix the problem.
I have been going through this for 5 months now with no resolution. Essentially they will not repair this type of issue unless they recieve a lot of similar complaints from people in the area regarding the issue. If it isn't an extreme slowdown, or as in my case, involves only the upload speed, an intermittant issue such as this might go unnoticed by most customers, or at least does not result in a lot of complaints.
See my thread below
http://forums.verizon.com/t5/FiOS-Internet/Slow-real-world-upload-speed/td-p/654849/highlight/true
Let us know if you are ever able to have the issue fixed and what did it.