It's possible that you have another, more subtle digestive allergy to some types of food. For example, people can be allergic to wheat, sugar, nuts or food additives. In order to really diagnose the problem, you'll have to neutralize your diet and start from scratch, slowly adding foods so that you can observe their effect on your body. This is difficult and can take time. But once you know which foods are causing the gas, you can eliminate them from your diet. There you go --- no more gas!
You can also see a nutritionist for help with this process.