I had a similar problem with servers dropping off-line but after ckeching the agnet logs I found that the agents were up and running, but couldn't connect to the Kserver. After some digging, I found that a setting in the NIC was the cause.
under the LAN interface, click properties > Configure > Power Management
uncheck the "Allow the computer to turn off this device to save power"
I found that Windows puts the nic's in a sleep like mode listing but not transmitting which causes your agents to go offline at the Kserver.
I hope this helps, if this isn't your solution then best of luck.