It would help to know the application. Also, unless you have been watching this server and have a clear history of what it has done and where it has gone, there isn't much you can speculate at.

What has changed? Added many more users? Database (assumtion) has grown 2+ times as large?

Been awhile since I've played with AIX but there are many tools included with it that you should be able to utilize to find what may be causing problems - but first you have to be sure you know that this "paging" is really a problem. Don't always believe it just because someone says so. It could be as simple as a user running reports that should be done during the night.

Check what's running on the system during these times.
Get someone to call when they see the problem - someone you believe to be truthful and who knows what they are talking about.
Become friends with your DBA (if applicable) - they can give insight to the application database.

Check on IBM's web site for more tools and resources.