How is your 5ghz coverage???
If you got islands of 5ghz this would happen because remenber that 5ghz has less range than 2.4ghz... if you or your partner did a survey in 2.4ghz for this WLAN then you might have this issue because as the client has not good signal from 5ghz it will mostlikely switch to 2.4ghz, and if you got band steering on it might try to switch back to 5ghz... and so on...
So check if where you having this issue you got a good 5ghz signal.
Cheers
Carlos