I have the same problem, I posted about it here before I saw this thread was already up. Like somebody else stated, stopping and starting manually appears to work just fine. Given that, I doubt it's SES that is causing the problem.
I have been having an issue where the interval setting for group mail will not work. It will send out the initial batch then begin the countdown, but once the countdown hits 0 it simply freezes.
The delivery information stops at something like this:
..and the settings page looks like the...