A standard procedure for situations like this is as follows:
1) Save your current settings (Administration - Restore/Save/Upload Setting).
2) Factory reset the router with initialise (Administration - Restore/Save/Upload Setting).
3) Do a minimal setup of the router and see if the problem is still present.
4) Restore your previous settings and test again.
But again, an important part of any testing is to disconnect all clients from the router other than the one you're testing from.
Of course none of this will matter if the problem is actually with your cable modem or your ISP's network.
I might be able to skip some of those steps as I think I narrowed down the problem to 1 (or more?) wireless devices;
Apple TV 4 in bedroom (wireless)
Apple TV 4 in living room (hard wired)
iPhone 8Plus
NVidia Shield TV (2019)* (wireless)
Previsouly, when the spike would happen, one of my iPhones showed up in the router logs (an iPhone 7Plus). I have since disabled wifi on that phone yet the problem continued. Next up (and the reason for the asterisk) was the Shield TV. I tried unplugging it before and it didn't make a difference but I unplugged it again along with two other Apple TVs and the new iPhone 8Plus and AMAZINGLY, I was able to do a ping test for a solid hour and never saw the spike!!!
The reason I started down this path was because of something I saw in the System->Wireless log: (ignore the Fire TV stick....left that in for reference)
Code:
Device Associated Authorized RSSI PHY PSM SGI STBC Tx rate Rx rate Connect Time
Shield TV Yes Yes -49dBm ac No Yes Yes 866.7M 702M 00:04:56
Apple TV Bedroom Yes Yes -50dBm ac No Yes No 866.7M 526.5M 00:05:14
Fire Stick Yes Yes -49dBm ac Yes Yes Yes 866.7M 6M 02:44:46
Roku Yes Yes -51dBm n No Yes Yes 117M 130M 09:17:20
Sorry for the bad formatting but I saw that some of the devices were connecting at the theoretical max speed of 802.11ac. (Who knows....maybe this is just the router saying...."hey, I'm connected at the max speed for some of your 802.11ac devices....") Maybe this doesn't mean that these devices are Tx at 866.7Mbps every 15 minutes but it got me wondering. So I disconnected the 2nd iPhone, the Shield TV and both Apple TVs and no more every 15 minute spike.
Now, I plugged back in the Shield and both Apple TVs and am getting the 15 minute spike again so right now, it's down to one of those Apple TVs I think. (Since powering off the Shield a while back in another experiment didn't fix things, I really don't think its causing the issue.)
But this leads me to question.... maybe my router can't handle all those 802.11ac connections? Because I'm down to one of those Apple TVs being the culprit. Whenever I find which one, I'll leave it ON and power the other one off and see what happens but I think I'm really close to being able to finally put this issue behind me.