Hi, did you ever get this resolved? I have the same problem. Late 2011 MBPro (8GB ram) running Win7 in Fusion 5.03.
I can run Unity to start with then it disables itself after a short time and I have to restart Fusion. Very annoying as I need to run Win for work. I have given Fusion all the system resource I can (e.g. use 4 processors) to see if this helps but no joy.
Thanks,
Terry.