My windstream dsl has been dropping every night as well for the past 5 years that I've been with them. They say they're working the problem but it never get's fixed.
I have no choice but to use windstream since they're the only high speed internet provider in my area otherwise I'd switch to a new provider.
I think the reason it drops is that their service in the area is just oversubscribed and they just don't want to update the system to handle the amount of internet users.
A lot of people come home from school and work and get online during the evening and night and when they do the windstream dsl speed drops like heck.