Slow internet and intermittent connections. Let's begin with 2.4GHz

My network is the strongest and consistent @ about -56 dbm, but...

I have several strong networks (I think that's what they are) that present on the same channel at -58 or so and they come and go every 15-20 seconds. To clarify, they alternate between -58 and nothing.

The entities have generic SSID's like "hidden" and Xfinitywifi. Some are open and some are enterprise security. I think this is driving my effective bandwitdth down but I can't figure out how it could be so strong for 20 seconds then completely gone for 15 seconds. Could it be something coming from my gateway? I've looked through my admin tool and I don't see any guest networks set up. I want to change channels but of course the gateway doesn't let you. Any ideas?