You mention that the problem is only limited to SOME computers, so my thought would go immediately towards your hardware and network equipment lacking.
So some things to troubleshoot that would be:
1) Monitor your network traffic on your filemaker host to see if you have the bandwidth for 80 users simultaneously connecting.
2) Check your network hardware for the same (IE is there network bottlenecks at switches and routers or hubs that are causing it to freeze up).
3) Check the opening scripts of your databases to make sure they're optimized.
4) Check the local machines for virus files, and check the ram usage while filemaker is running (I've seen filemaker be a resource hog before).
5) Do the machines having issues have a commonality between them (IE operating system, lack of java updates, only 1gb of RAM, etc..) and are they MISSING anything that system users with no problems have.
If it was all 80 users having problems I'd be suspicious of a filemaker problem, but I think this is most likely hardware based.
Thanks for the answer...
But the main issue is when some compuerts works under domain configuration
I test the filemaker connection over internet on my client's network and while my computer was outside the customer's domain, i can connect to my DB.
But when I put my laptop under client's domain, then the network communication failed again.
Any Ideas why a domain can affect the FM comunication between client and server?
something along the line in the network (IE group policy or firewall policy) could be blocking ports for filemaker traffic. Some of the stuff I listed above is still applicable for LAN vs. WAN testing.
Here's the list of ports to test/look for: