Network lan serious problems with two dvr

I have two dvr one avc799zb and 16ch h.264 bd / 98 work correctly ie tri-fold display all cameras in the dislay, are connected to the LAN to turn on the DVR via the web and can be accessed VideoViewer cameras. So far so good the big problem is that after a few days longer accessible web so as VideoViewer having to turn them off and on again to recover this function.

What would be the problem?

and what the possible solution?