Things like this are soemtimes self-healing.  It could be the ISP doing maintenance or some other like operation.
It could be a legitimate problem.  One they may, or may not, be aware of.  It is always best to assume they have no clue and send them the information showing the problem exists.
I haev already sent a report to AT&T about it.  I suggest you hit up your ISP from the other side as well.