This is getting to be an interesting discussion. To David Leheny: your observation about the impact of complexity is right on target, and--this is a compliment--I think it is the same perception that led March, Olsen, Michael Cohen and those other smart guys around Stanford to think up the garbage-can theory. I think you would find it very interesting to look at the original essays, some of which were collected in James G. March, Decisons & Organizations (Blackwell, 1988).
To Prof. Hiwatari, let me suggest the recent book by Frank Baumgartner and Byron Jones, Agendas and Instability in American Politics (U Chicago 1993). It is a "puctuated equilibrium" model that tries to explain both stability and change in policy, with a lot of pretty systematic evidence.
Let me not go beyond this bibliographic response at the moment--I have been plagued for so long by so many of these issues that I would just write on endlessly.