Oops, I spoke too soon. It just took longer, but using the AU plugin, it also went to sleep. I'm using two instances in VEP7. One instance has 10 Synchron player instruments and the other instance has just 1 Kontakt instrument, but it has 12 articulations loaded.
And just in case you are wondering, hot corners are turned off. When it happens, the computer becomes completely non-responsive until it is fully asleep (silent, no fan noise, etc.). Then, and only then, I can wake it and everything is still open and working. However, it starts happening more and more frequently once it happens the first time.