Based on the frequency graph, you've got your R710 sitting at the upper end of the DFS range. Try using the low band (36-48) or non-DFS high band (149-165). Despite higher signal power, you may be taking interference from a RADAR system or from other APs nearby in the same band. I would be curious what your retransmit statistics are, but that's understandably hard to get from an iPhone.
This is assuming you are in the US. DFS bands may/will be different world-wide.
This is assuming you are in the US. DFS bands may/will be different world-wide.