For the ribs, cut each rack into halves, if desired. Combine the paprika, cumin, salt, garlic powder, onion powder, pepper, thyme, and oregano in a bowl and mix well. Rub or pat the spice mixture over both sides of the ribs. Place in a large shallow baking dish. Chill, covered, for 2 hours or overnight.
For the sauce: heat the butter in a saucepan. Add the onion and saute until tender. Add the garlic and saute for 1 min. Stir in the ketchup, molasses, brown sugar, Worcestershire sauce, vinegar, mustard, thyme, liquid smoke, salt and pepper. Reduce the heat to medium and bring to a boil. Cook until thickened. Let cool.
Pre-heat the oven to 300 degrees. Arrange the ribs in a single layer in a roasting pan. Bake for 2 hours or until the ribs are cooked through and tender. Brush the sauce over the ribs during the last 30 min. of baking.