First thing I would do is cut out any foods that are highest risk to cause these kinds of problems (including chronic inflammation):
Grains (wheat, corn, oats, millet, bulgur, rice, etc.) - Gluten and other grain proteins can wreak havoc on the GI tract. This is number one, very important.
Dairy (milk, cheese, yogurt, etc.) - Some people are very intolerant to dairy
Legumes (beans, lentils, peas, soy) - These have lectins (antinutrients) that prevent you from absorbing nutrients (as well as other problems)
Industrial oils ("vegetable" oil, canola oil, corn oil, soybean oil, sunflower oil, safflower oil, cottonseed oil, basically anything that isn't cold-pressed olive oil or coconut oil) - These are full of pro-inflammatory Omega-6 oils, and are highly oxidized (which is bad).
If you cut those out for a month I can practically guarantee an improvement in your situation. So what can you eat? Meat, vegetables, nuts, fruit, avocados, herbs, spices. Basically a Paleo-style diet. Check out the book "The Paleo Solution" by Robb Wolf, he recounts his struggles with a similar situation (diarrhea, losing weight, not digesting food).
I would also suggest taking fish oil every day as well to help combat the inflammatory response. 3000mg/day of EPA+DHA should do it (comes out to about 10 pills if they each have 300mg).