You should press your host until they give you a solid answer as to whats going on. If they cant figure out why you are pushing more traffic, how are they going to handle a situation where your sites are down or not working?
We have monitoring in place at
www.realitychecknetwork.com that allows us usually to detect these issues before our customers and analyze the situation and provide solutions right away.
Its in a host's best interest to keep their customers happy and if you aren't making money from the traffic and you are working with them they shouldn't charge you for the overages, or at least work out a huge reduction.