Lost database connections

Our configuration is IDS 9.21UC7 on a Sun VX880 server running Solaris 9. This server is running in 64-bit mode although our database is 32-bit. We have clients connected running a Power Builder application. Since upgrading our server from a Sun E-450 on Solaris 6 we are experiencing loss of connections when the clients are idle for approx 20 minutes or more. Does anyone know of possible issues/checks that can be made in either the database or operating system that can effect dropped connections ? David Fortune