Is this a new setup? If not, when did you upgrade the software version?
Did you had the same issue in previous version?
How may R750 APs are having this issue?
Is this observed on other models as well?
Do you have Preferred Master set?
We might need the Himem logs from the AP which are facing issue, it would be great if you can create a support case on this so that the TAC team can assist you on this.