Large ping times sometimes to CPE's

I have a RB433AH with a 5.8 link and a 2.4 Omni AP traffic is routed out ethernet port , there are at most 40 CPE’s connected to the omni with on average a max 10 active at any one time. The problem is that some times the ping times to CPE’s go as high as 700/900 which seems crazy , some clients have complained about slowness at these times. I have looked at ack timeouts and CCQ but can’t see anything obious but perhaps I’m not looking for the right thing, any suggestions folks