I feel your pain! I'm 24 and have been suffering with GERD for the last 10 years or so. The IBS started for me about four years ago, before I was even prescribed medicines. My IBS is usually due to my lactose intolerance and not eating proper meals.
Usually if I have the cramps associated with diarrhoea, i'll just use a home remedy of gelatine based water, which in turn kind of makes me constipated. I just make sure to eat a fiberous diet, and lots of fruit that's not too sugary.
You might want to switch medicines also. If the tagamet isn't working properly (which it sounds like it's not), i'd suggest Zantac. I take 300 mg of Zantac when I need it, and it hasn't seemed to bother my IBS much. I was prescribed 300 mg by my dr, so start off on the 75's.