Generously salt and pepper ribs. Lay out ribs in a single layer, in roasting pan. Cook ribs for 3 hours, turning once midway through cooking time. Remove from oven and cool slightly. Pour off any fat from bottom of pan. When meat is cool enough to handle cut slabs into individual ribs.
While ribs are cooking, saute onion and garlic in olive oil in a large stockpot, until soft. Add remaining ingredients, except molasses and guava paste/jelly and bring to a boil. Reduce heat and simmer, stirring occasionally, reducing liquid by half, approximately 1 hour. Sauce should be thick, but still loose enough to pour. Add molasses and guava to sauce and continue reducing for 10 to 15 minutes until thick enough to coat the back of a spoon. If sauce becomes too thick, thin with extra guava nectar and/or beer. Season with salt and pepper.
Coat ribs generously with sauce in roasting pan. Continue cooking ribs for 1 hour more, or until rib meat is soft and easily pulled from bone. Sauce will thicken and darken on ribs, so be sure to turn darker ribs over. Before serving, brush ribs with additional sauce and serve extra sauce on the side.