Lightroom :: Crashing During Move File And Other Processor-intensive Operations?

I've been using various versions of Lightroom on my Mac for many years without issue. I love the program. Recently though, when using the last iteration of Lightroom 3 I began to experience problems when performaning processor-intensive work. For example, my library has nearly 200K images, and these are organized into hundreds of folders (bird species), each folder containing from 1 to up to 14K images. My workflow is to bring in the photos, rename them, and then shift them over into these species folders. For me it's an easier and faster way to get my hands on specific images than using tags. In any case, when I try to move images between folders lately, Lightroom slows down and eventually crashes. It also crashes the other apps running on my Mac at the time (usually just a web browser). I get the gray screen of death and have to reboot the Mac. Start over and after a few minutes of work it happens again.
 
Today I upgraded to Lightroom 4.3  hoping that might solve the problem, but I experienced the same crash behavior tonight while shifting some photos around. The Mac is a Macbook pro, recent model with latest OS and with 16GB RAM. I don't experience problems like this with any other application, but Lightroom is now bringing my computer to its knees! I even took the Mac into the Apple store and had the full diagnostics run and they found no problems internally. I keep my photos on external drives connected by firewire. Another potential wrinkle: Lightroom doesn't seem to be able to find a few of my previously imported images even though the directory is looking for them in the right folder. I get a "File offline or missing" issue. I tried pointing the folder to a new location (the same one) but that didn't work.  Oddly, it does seem to find others.