Every O2 sensor in Renix and 91-92 OBDI MJs are "pre-cat". If it's bad, you stay in open loop. Bad mileage yes, but most always no miss. What the OP has is an intermittent ignition cut off problem with the tach shutdown. Most likely causes are a bad CPS or it's connector problem, a distributor pickup problem, a failing coil, or a grounding problem as Cruiser detailed.