My 1990 Jeep Cherokee Laredo, 4.0 liter sometimes will not start. there is no spark from the coil to the distributor cap. the problem only happens sometimes when trying to start the car. after letting the vehicle sit for a day or 2 it will start again and run great. the problem has never happened while the car is running. the starter system works perfectly. can a coil fail intermittently? what else could be wrong?