I had the following problem with PM recently, I thought I'd share it as it my be useful to other users.
I'm using PM 4.6.6 on a 2.16 Core 2 Duo iMac with 3GB of RAM running OS x 10.6.6.
When I opened one particular folder on my hard drive that has 400 NEF files everything was fine when using the contact sheet and opening the files in NX2. But when I went to preview mode to zoom to 100% to check focus everything was fine for the first minute or two but then the PM hung, I got the spinning beach ball and when I check the activity monitor the PM process was at 101-103 in the CPU% and stayed there
At this point I had to force quit the PM.
I did not think this was really a PM problem as other folders are fine and program works in it usual brilliant way.
I'm guessed there was one or more file corrupted.
When I scrolled down through the images one by one in the contact sheet I came across one file with no preview. I opened this outside of PM in NX2, made a quick change and then saved to update the inbuilt jpeg.
Back to PM and everything is fine, no more spinning beach balls.
I'm guess the preview got corrupted when I batched applied some Nik filters to the images.
John