First thing I would consider is going to the latest fixpack. The level you are on is nearly 5 years old. I would look to becoming current. Second, V9.1 will be going out of server soon, so you should look to upgrade soon also.
There are some issues for using 64K (medium) pages for shared memory on AIX platforms
by db2 v9.1。
the command "vmstat -P all" will show you whether 64k pages are available on your system.
Try db2set DB2_MEDIUM_PAGE_SUPPORT=OFF and restart instance to see whether it will happen again.