When AFP is not running at the server, Mac OS 9 clients that try to use NetBoot process will display a gray screen, pause,
then possibly start up from a local volume.
If this happens when AFP is running, be sure that none of your network hardware, such as a switch or router, is interfering with AFP traffic. A simple test for isolating your network hardware is to run a crossover cable between a Mac OS 9 client and the server.
Be sure that any switches and routers have the latest firmware that is available from their manufacturers.