What might be the cause of regular periodic slow transactions on an OLTP system?
High Log File Sync wait events mean that throughput on writing to the online redo logs is not high enough. This blog gives a great explanation of how to diagnose and troubleshoot that event:

http://logicalread.solarwinds.com/oracle-log-file-sync-wait-event-dr01/#.V_TSTvkrKUk

GC Busy and other GC wait events indicate RAC cluster related waits. We'd need to see a lot more detail to troubleshoot this accurately. That said, if you are having Log File Sync waits on some or all nodes, most likely the other nodes will start showing GC wait events. The nodes have to cooperate with each other. When one node has problems, the whole cluster is affected.