If you are using Revoice Pro 1 or 2 on OS X 10.9 or later you need to disable App Nap.
App Nap freezes applications to save power, however this can cause your host DAW to lockup when it uses the Revoice Pro plug-ins.
To disable App Nap select the Revoice Pro application show the context menu and select Get Info
Make sure that Prevent App Nap is selected.