I'm using Nessus 3 on Windows XP and I have a class C range to scan. I've run the scan with all (safe) plug-ins enabled. I have gotten different results every scan with various warnings and holes showing up and not showing up. Is there a way to be more accurate with nessus (short of using another scanner)?

Yes, all the addresses are static. I'm not scanning an internal network, I'm scanning the external IP range for our company. I'm thinking it could be something with the firewall since all the traffic is going through it.

Yes, the NATing is done one-to-one. We do have a load balancer, but I'm not 100% sure where it sits in the architecture. I think I'm probably running into the issue with the firewall answering for servers that aren't really listening as you've mentioned. I'll check into each of those. Thanks for the ideas.