Using The Dgl-4500 With The Dgs-1016d Switch

We have both a DGL-4500 and a DGS-1016d. We connect to the internet via a Mototorla 4G broadband modem from Clearwire. The connection goes like this: Internet ==> 4G Modem ==> Linksys VOIP box (SPA2102-R) ==> (WAN port in) DGL-4500 (Port 3 out) ==> (port 5) DGS 1016D ==> various ports for PCs, PS3, Echostar satellite boxes, etc. About once a day the connection to the DGL-4500 goes dead. We see no light on Port 5 of the DGS-1016D (which goes to Port 3 of the DGL-4500). All devices downstream from the DGS-1016D lose their connection. There is 1 PC which is connected directly to the DGL-4500 and it retains its connection when the DGL-1016D connection dies. The VOIP box and the 4G modem continue to function normally. Do you know of any reason why this might occur?