heh, Al was doing the SAME thing. it was freaking my girlfriend out. he'd throw up during the night, and then in the morning a couple times.. he did this for several days straight.
I figured he probably was either eating too much and throwing up, or his stomach was too empty and he was throwing up stomach fluids. So, what i did was instead of feeding him two normal sized meals during the day, i switched to feeding him a half meal in the morning, a normal meal after i get home from work, and a half meal in the evening before bed (just enough to help push that last BM of the day).
That seems to have fixed it.