There were a lot of very good and interesting suggestions, but the first answer I thought of when reading the original question has to do with the garlic. Be sure you don't over cook your garlic as this will definately cause a bitter taste. Also, the purpose of the simmering of the sauce is to carmalize the sugars in the tomatoes and this is usually done over a period of several hours, but several days? I have to wonder if there is another chemical reaction going on, causing the off taste. Like Regina said, if you want to thicken the sauce, use tomato paste. As far as the baking soda, it does neutralize the pH (reducing acidity) which will help those with stomachs sensitive to spice