The PostgreSQL's log file is already giving you a hint that you have to tune your PostgreSQL. You have to increase checkpoint_segments, wich is pretty common in OpenACS installs. Concerning your Hardware, I would say you should try to increase to 6 this value, but I don't know if your system can handle it. Try it and see if you can see some improvements.
I would say in a general way that you have to do some tuning in your PostgreSQL. I can see that you've increased shared_buffers, and maybe that's the reason for wich you are facing this kind of trouble. You see: both PostgreSQL and AOLServer use shared memory, controle by the shmmax parameter in the So Kernel. If you use a too big value for the PostgreSQL shared_buffers, it can start a race for the same resources with the server, causing the system to crash. It already hapenned with me.
However, I don't believe this is the problem in your case. Take a look at the queries being executed at the crash moment (usually a select * from pg_stat_acitivity would do the job) and try to find out what is happening at that time.