Galera Replication Utilizing Percona XtraDB Cluster - Load Balancing Method

I've no experience with setup I'm afraid but Least Connections is generally preferred over RR. In this scenario Observed or Predictive may be more worthwhile but I'd suggest you read up and test appropriately. Here's an overview: http://support.f5.com/kb/en-us/solutions/public/6000/400/sol6406.html?sr=31786154

Are you sure the spikes are caused by uneven load balancing? What's the evidence for that?