Re: 404 errors turning into 500 errors through internal redirects
Just a couple more thoughts while I think about your situation.
Have you determined the offending party that keeps triggering the redirects? Is it Googlebot-image? Have you disallowed it from searching your site with a robots.txt file?
If so, then it probably is not actually the Googlebot-image spider but a spam spider attempting to masquerade as the Google image bot. A robots.txt file cannot stop a spider from crawling your site. Reputable search engines honor the robots.txt file requests, unscrupulous ones ignore it.
For your information:
Are you using Firefox? With Firefox and the Web Develoepr Add-on, you can get a lot of information about errors.