To test, disable UPnP on the router and enable DMZ to your PC or console and try gaming for a while to see if you still get ping spikes. If you do, there could be some quirk incompatibility with modem and/or ISP (would have to test using Ethernet), or simply something with the WiFi connection/quality. If not, could be another issue possibly with UPnP on the router.
Definitely disable Smart Connect and do not use QoS.
Does this happen when connecting to the router using Ethernet?
Make sure your modem has latest firmware.
Do you have issues streaming video (netflix/hulu/etc)?