Network devices locking up

I hope someone can offer some advice on a problem that has been driving me mad for weeks. I have a wifi network with 30 access points 2 poe switches an access controller and a dsl router. The 2 switches are linked via fibre. The problem I have is that the system works fine for upto a week and then a series of devices lockup and are unreachable on the network. It could be an a couple of access points or even the switches or access controller. It really is very random and I can't replicate the fault. I've replaced the switches and access controller so I know it's not them but I can't seem to identify the fault. I've already rebuilt the network by adding devices 1 at a time and checked with a mac scanner to make sure there are no conflicts. I'm running out of ideas so any advice would be great..