This could be a million things:
* The cell phone is going into WiFi sleep mode (more specifically known as "Fast PSP" and "Max PSP" modes) because of low data throughput and is checking in to get its data more and more infrequently. This is usually not configurable n a phone. It could be affected by battery remaining, and especially the current amount of data throughput.
* WiFi spectrum can only be used by one device at a time. Perhaps some other device - not even on your network - is using the spectrum reducing the amount you can use. Slow devices make this much worse. I would recommend making the minimum data connection rate 12Mb/s to disable old slow data rates in your environment. You could also check channel congestion by going to Wireless/RF Spectrum.
* Perhaps you are close enough to something operating a radar to be detected by DFS and the AP is responding to that.
* Perhaps some other general device like a cordless phone, microwave, etc, is causing interference.
