Modem or router problem?

Hello all,

My TP Link router crashed. Brick kept restarting every 10 seconds. So I went back to my old DLink DIR615. The modem from Cox is a Cisco DPQ3212 from 2013.

Since I did that, my internet keeps going out every 5 to 15 minutes. I think it's the router re-booting, but not sure if it's the Cable Modem. So, that brings me to a few questions. 

1) How can I tell if it's the modem or router? I've reset and tried a few things with the router settings, but no luck.

2) If I need new hardware, would it be better to get a docsis 3.0 modem/router combo, or try a new router only?

Thanks for any tips,

Doug B