Why are there periodic regular slow transactions

I have a two node RAC cluster with that runs an oltp application against it.  I have both a production and a duplicate test environment with the same setup. Upon running stress tests on the Test environment (about 160 tps), The average latency is about 70ms.  I noticed that when I examined the maximum transaction time within each minute, that there is a regular spike in the maximum transaction time every 5 minutes of several transactions that take about 1200-1500 ms.  I've traced the individual long transactions down to the second  and them seem to last for anywhere from 3-5