first thing that comes to mind is, "can you go fine when no-one else is around or do you still have the sphincter lock up tend to happen as you go?" its just it sounds like you've got shy bowel... but you may also have anismus (see biofeedback on the things that can help page of this site). worth looking into it if just to eliminate it (can't avoid those puns) from your enquiries.
you also said you've been searching the internet with little success for weeks. i'm hoping to improve the visibility of the site by finding out what the INITIAL searches people use when they are hoping to find something about this on the internet. if you remember all or any of the searches you used (especially the ones near the beginning of your search) it would help others find the site in future as i can get john the webmaster to put some meta tags for google to pick up.