Just a quick note in case someone else has this problem.
After upgrading my production VCenter Environment to 6.5, we were experiencing a high number of VMotions between hosts. It was like DRS had lost it's mind. Our clusters were never going above the Standard Deviation value, yet VMotion would occur constantly. My primary cluster with 22 hosts performed approximately 20,000 VMotions within 2 weeks. That cluster had only performed about 3,000 migrations in the lifetime of the cluster prior to the upgrade. We bumped the DRS Migration Threshold from 3 to 2 with no real change in behavior.
In the end, it looks like there have been some substantial updates to DRS in 6.5. You can read about them here:
The feature that was killing us though was Pair-Wise Balancing. By trying to keep all hosts at roughly the same performance level, it was migrating VMs constantly. It doesn't seem to affect my clusters that only have a few hosts, only my clusters that have 6 or more hosts.
To turn the feature off, set the following advanced option in your DRS settings for the cluster:
CheckPairwiseImbalance = 0.
Support wasn't overly familiar with the DRS updates in 6.5, so they had some problems assisting me. We ended up finding the issue ourselves, so hopefully this helps someone else out there after their upgrade.