The larger the rib, the tougher. Secondly, you may be using too much heat. At this temperature it will take 4 hours or more to get them really tender. They are done when the meat starts coming loose from the bone. Don't forget to use a mop sauce on them during the cooking process.
Mountain oysters are the testicles of animals.
Most commonly they are obtained from cattle, hogs and turkeys. The most popular way to prepare them is to soak them in cold salted water and then remove the membrane covering them. They are then sliced, battered and deep-fried. They are a very popular dish with certain segments of the population. Here are a couple of recipes that use corn, a historic Texas staple.