Nial,
If you want to use parrale, you will experience this kind of behavior. I just don't understand why the people will not just let Windows use all of the resources from their computer by allowing ONLY Windows to take complete control of it. When you use Parrale, the Mac has to drive two operating systems, Parrale and Revits simultaneously. If you make a living with Revits, we don't need these unnecessary problems.
Boot camp is for free. Boot-up into Windows and let Windows use your resources efficiently. Revits is huge and the rendering uses a lot of resources. If you want to use OSX, boot-up the Mac while holding the option key.
I have Mac Pros and several Macbook Pros with no problems at all. Business is business! We cannot play around with two LIVE operating hogging all of the resources generating erratic behaviors.
Please remember that this is just my option and only my option because I have only been using CAD machines since 1989: Look up IBM PS-2 Model 60.
If for whatever reason you don't want to use Boot Camp, good luck and I hope the very best solution for you will be obtained.
--Al