Just a thought:
All the server upgrades in the world and likely not the client upgrades will help if there are issues with bandwidth. It sounds like you know what you are doing so I don't want to be condescending. Most major issues with AC are reported by many users, and they are known bugs.
On the weekend, I would come in and use one computer for an hour, then add another, then another. You say it is in different computers. That sounds like bandwidth. I would check the router or switch first. Make sure they are GB. The ethernet cable from the server to the switch has to be GB ethernet. Check the cards on each computer and make sure they are autonegotiating at 1GB.
To be fair, this doesn't sound like an AC issue. It sounds like a network issue. If you can get a certified IT person to come out with a fluke meter and test every connection for $100, you may get to your problem rather quickly. It only takes one connection. Get a managed switch so you can see what the speeds are. Or get a cheap switch that is definitely 1 GB and add one at a time.
