Something strange is going on when I render where my dielectric materials are turning black. It happens randomly (I think) and sometimes midway through a render. If I save the image as a .psd the black areas inside my dielectrics (martini glasses in this case) are transparent in photoshop.Here is the image http://imgur.com/qqpHL
The nearest glass is fine except in the right corner of the glass. The reflections in the table appear fine. Also I've noticed that everything looks fine when the render is in it's first phase (where it goes from blue screen to fuzzy image) and usually screws up in the second phase where it refines the image. Any help here would be much appreciated.
Maya 2011 64bit
Intel Core i7 CPU 930 @2.8 GHz
ATI Radeon HD 5770