Shared memory oddity

Why would this happen?

I had a server where postgres appeared to be running very slowly. Following
some advice I found on-line, I boosted the shared memory max for that
machine and told postgres to take 15200 buffers (about 128 meg of shared
memory). For a while this worked great, but today, the system runs very
slowly again.

free and shared memory look like this:

[slane@iep slane]$ free
             total       used       free     shared    buffers     cached
Mem:        513940     511652       2288     295248      38100     371896
-/+ buffers/cache:     101656     412284
Swap:            0          0          0
[slane@iep slane]$ ipcs

------ Shared Memory Segments --------
key       shmid     owner     perms     bytes     nattch    status

------ Semaphore Arrays --------
key       semid     owner     perms     nsems     status

------ Message Queues --------
key       msqid     owner     perms     used-bytes  messages

[slane@iep slane]$ cat /proc/sys/kernel.shmmax
cat: /proc/sys/kernel.shmmax: No such file or directory
[slane@iep slane]$ cat /proc/sys/kernel/shmmax
160000000

Postgres used to have a big section of shared memory in use. Today, nothing.

postgres 7.1, running on LinuxPPC on a PowerMac G4.

Any ideas?

-- sgl