Random Slowness with Virtual Machines

[actadmin]

I have 20  ESXi 5.5 hosts and i'm seeing a strange reoccuring problem lately.  User's will complain about extreme slowness on certain VM's. When i migrate them to a new host the problem goes away untill i migrate them back to the original location the slowness starts up again.

This points to the actual HOST but i'm stumped on what to look for. All system configurations and resources look normal.  

Anyone have ideas of what i could look for or where to start troubleshooting this issue?

[BenLiebowitz]

Can you provide some more information?  What hardware the VMs are running on?  What storage is in the background?   How many VMs are running on each host, esp the one(s) where there are problems?