There is the following query tuning guide that may help:
http://www.aerospike.com/docs/operations/manage/queries/
Its possible that with load your server is reaching a certain network bottleneck.
Is your network using 1G or 10G ? Whats the network usage during these high load period.
You may be able to use a tool like iperf to test your network during load.
https://discuss.aerospike.com/t/iperf-network-performance-measuring/729
How many CPU Cores does these nodes have?