I had EXACTLY the same problem... air/oil in coolant. Get someone to check it is exhaust gas, and not just air. Mine was just air. We pulled the rad, and found the cause. The rad was blocked, and trhe resulting vacuum in the cooling system had sucked oil in from the oil cooler, and air from outside (expansion tank cap - change yours!). Changed the rad 1500 miles ago, and now theres no oil or air in coolant.