I once likened IBS to erectile dysfunction. And look how long it took them to cure that (and I daresay men were working REAL hard to get a cure for it). The problem with IBS is that it doesn't have one singular cause. Maybe it's hormones, maybe it's anxiety, maybe it's mixed signals from the brain to the intestines. And that doesn't count the parasites, bacterias, yeast, bad gall bladders and appendices and everything else that cause bowel problems.
Are you taking imodium everyday, or just when you go out? Once a week ought not to hurt you. I can't remember if it is Lil or Des who used to use imodium heavily and eventually went from having D all the time to A, and the doctor thought long-term usage of imodium had caused the shift in symptoms, but I seem to recall she was using them daily for years before anything happened.
Have you tried the calcium? Even if that only helps you out some, it may make your imodium use a less frequent thing and thus lessen your chance that it will affect your intestines. I told the last G.I. that I went to see that I was taking one or two everyday for two months to control the diarrhea and she said, "That's not that much." Considering I had never before been able to take more than 1/2 of one without getting severly constipated, I thought that sounded like a lot! She never told me to stop them after a while or take less or anything. I eventually found the calcium and that went a LONG ways towards straightening me back out and I was able to drop the imodium.