I have a friend that bought a used 2014 RT SM6 in the spring with 6000 miles on it. He has put about 250 miles on it so far. He started it up several days ago and it started missing or skipping at idle. When he revs it up to 2000 rpm or more, it smooths out. It won't die at idle but does the missing, skipping sound. He did check for bad gas and the gas is fine. Battery checks out OK, too. Any ideas?