CM500 upload light flashing green

Hello. I have a CM500 (V1.01.12) which has worked generally well. Just recently, I added a Nighthawk Range Extender. I'm not sure if the two are related, but I think around the same time, my internet periodically drops multiple times per day. What happens is the upload light on the modem will start to flash green (at whcih point internet stops working), the internet light on my router (linksys WRT3200ACM) will indicate there's no internet. I unplug the modem and replug, and everything works fine, for a while anyway. I will say the problem didn't start immediately after adding the range extender. Comcast said everything looks fine.

Thanks for any help you can offer!