1984 Lincoln Town Car Engine cranks / won't fire sometimes

Engine cranks good but does not fire at all. If engine already running, engine will die and do the same. So engine can be hot or cold. But let sit for a day or so and car will start right up and run perfectly for maybe a month or so. then the same thing happens. By the time you have it towed to garage and they check it, it will start and run perfect.