Hi Andrew!
I have exactly the same problem with my mpb.
I got shutdown messages every 5sec. And after trying reinstall OSX my laptop doesn't start.
I can start only ASD EFI, but ASD OS not.
I disconnected my HD and battery, but laptop still doesn't start :(
Only when I've found your post I tried to start with Fn pressed and I've got ASD OS started!
Uhhh...
Now really I doesn't understand what's up?
Did you find out anything?
Why it's happend?