Excellent. My plan is working flawlessly. OP, I wouldn't go to sleep.

The assassins are randomly generated once you reach a certain level and will only stop attacking you once you join the Dark Brotherhood.
See the http://www.uesp.net/wiki/Skyrim:Assassin for details.
Alternatively, I think destroying the Dark Brotherhood puts the kibosh on the random assassins as well. Used to be my character couldn't walk more than five feet outside a city without running across an assassin. Did the "destroy the Dark Brotherhood" quest and haven't seen one since.
Now all I have to deal with are the Thalmor death squads.
