WordPress 3.5 HTTP error on most image uploads

Since I uploaded to WordPress 3.5 a few weeks ago, I can no longer upload most image files (JPEG and PNG). This does not appear to be related to the image size. All my images are 500x500px or less. Many are exactly 500×500. Of those, about one out of every eight will upload. The rest result in “HTTP error” in a red strip at the top of the media upload window. The image itself IS uploaded (I see it in the Media Library) but none of the “crunching” phase has been done. There are no thumnails generated which results in only the full size image being offered for insertion in a post or page (no thumbnail, medium, or large). There are no errors in the PHP log and only “500 Internal Server Error” in the server log. From other posts on this same topic, I believed this to be a memory error. Our initial PHP memory_limit was 128M. I doubled it to 256M and no change. I doubled it again to 512M and still no change. In my past life, using WP 3.4.2, I uploaded thousands of JPEG and PNG files and *never* had an error. I disabled all plugins — no change. This sure feels like a bug in the new media uploader. I have seen many posts on this topic and no clear resolution.