I like having the feature set available, if I am running a game server, plex, or VPN. But yea, a regular router/firewall is not off the table. I am contacting my ISP today about possible issues with my modem--though I haven't had any in the past. Last night I configured a Netgear VPN firewall, along with DHCP on LAN and some static entries/reservations. Same thing happens... works for a time and then everything goes down--seemingly for no reason.
My focus has sort of shifted to the modem; the NICs on the Jetway board check out.