All the fellows have great advice, but if this just all-of-a-sudden started, I would look for the simple things first. Cracked distributor cap, or mag cap (whichever your running) spark plug, like some of the guys said. Something dumb. You say it's a show tractor. Is the engine fresh? There's so many things that you have to take into consideration. If it's a freshly done resto and the engine is fresh, I would have to think it's something simple before I pushed the panic button. To me it sounds like it's an ignition problem. Something is causing the ignition system to go to ground and knock two cylinders out on ya. Run through the ignition system slowly. Check the coil (if equipped) for grounding to the high tension tower. You might be able to find the problem fast in the dark while the engine is running. Find an arch, or a spark, and there's yer' problem...
Good Luck! Hope you make that show!! Steve B.