I am wondering if your SD card is corrupted somehow. I think the Sigma raw files contain a JPEG and that is why the camera can "see" the image. One thing you can test is try viewing the raws in slideshow mode, which I think also accesses this jpeg. If you are on a mac you can try (download the demo) of one of the other raw converters to see if they can access the raw data.
One other thing, do you have latest firmware installed? Sigma reportedly fixed some file writing stability in certain circumstances that you might be operating within.
I thought of one other thing you can try, copy one of the "unresponsive" raw files and place it into a new folder by itself and load up Sigma PP and browse that folder, if the file previews okay, then it is a memory management issue with Sigma PP and your computer.
This is all I could find of a google search for your specific issue:http://www.dpreview.com/forums/post/51255868