CM1150V voice cable modem and Nighthawk AC1900 Smart WiFi router

that is a good questions. Usually you need to start at square 1 and rule that out. Square 1 is the modem. so do this with a hardwired pc: 

1. Remove the router. Directly connect your pc to the modem and use it. make sure you use it long enough to verify if it drops or not. 

Also log into it and take a screen snip of your modem connections page. Comcast might tell you the signals fine but that's usually their go-to response to get people on their equipment or keeping from having to send a tech out. 

2. If the modem checks out and is stable for an extender period. Then we move to the router. 

What firmware is on the router?

If its version 1.0.2.8, I'd start with a factory reset. In that version, netgear updated the router to include netgear armor and they also removed some features. It was a significant change. Enough that the routers have responded well to a factory reset and re-doing your setup manually (not using a backup).  If it doesn't work, try downgrading prior to netgear armor. It hasn't been the smoothest rollout. Also reset the device afterwards if you downgrade. 

Try the modem first and then move on to the router. If the modems stable and the firmware resets, downgrade doesn't fix it, let me know and we'll move on from there.