During the last couple of ESX 3.5 and VC 2.5 implementations I encountered some weird DRS/HA behavior:

During the reboot of a VM ESX decided to vmotion the VM, as you can imagine this took a lot longer than normal due to the memory being excessively active.
When the DRS Affinity Rules in a two cluster node […]

VMwarewolf already posted this fix on his blog but had to remove it… Now VMware added it to their knowledge base. Check out the original article because it may change in time. For the lazy people I included how to diagnose the problem and more…
Diagnose the problem:

Use the VI Client to log in to VirtualCenter […]