I havea list of things to check when Image stops working.
1. On exisitng machines with agent, does task Agent exist, what task server is it pointing to. If the task server is not the one it should connect to, fix that in Sites or Configure SBS to replicate to all site servers.
2. Does the machine boot PXE manually. If not check the Netboot services on the PXE server.
3. On the task server/pxe server are all 4 netboot services running.
4. Does that task server receive the SBS files for setting the netboot profile.
5. Pray.