Hyper-V servers tank when VMs use more than 150gb out of 288gb of RAM

I am having a problem with two Hyper-V servers (Win2k8R2 Ent).  They each have 288Gb of RAM.

The problem is .... when the VMs collectively use more than about 150Gb of RAM, the servers grind to a halt.

Network connections drop....VMs freeze or run extremely slow etc.

The problem does not appear to be CPU related...it is totally memory....if I give one server an extra 5gb RAM, the next day the whole system will tank out.

Any ideas?

I also have problems with VMs failing to shutdown...they get stuck on "Turning off" indefinitely, until i restart the server.