To start with, you shouldn't be having those kind problems on a car that new & with that few miles. The problems you should be having would be worn tires, brakes, belts, air & fuel filters, wind shield wiper blades & depending on what kind of car, it might be time to change the timing belt. Every thing I've mentioned so far is considered to be general maintenance except for the timing belt. Many cars don't even require a tuneup for the first 100,000 miles. You did good by getting all the repairs covered by the warranty. Especially the bad manifold. If you start having any more problems anytime soon, you should check your city's lemon laws. Oh, about changing the spark-plugs, they're usually good for at least 100,000 miles but they are often replaced when you get a tuneup. I would suspect a bad connection for causing the bad coils.