what does the maint logs say when this happens? When I have this type of issue I try to narrow it down to one of the devices (as it could be either causing the issue). So one fothe first things I try is; put the keypad on a static reservation, put the PC on a static address (or reservation) then when the issue next occurs, from the controller attempt to ping both addresses. In most cases one of them has dropped off the network so will not respond. From there you can start to dig a little deeper but you know which device has the issue.