Ben, Head over to this thread:
Looks like others are having similar issues, lets see if we can gather some data, and figure out the issue.
i don't think my symptoms are similar to the OP's on that thread. Although I do think I have had a similar problem a while back.
I'm familiar with processes to isolate problems like this but i haven't dedicated the time to documenting the exact circumstances of each time i have the problem and how i eventually get it to work. it all just seems so random.