Got me on need to change plugs out. If glow time on cold engine is within specs then i believe you describe a normal occurance on most diesels. They start and run better with two cycle inputs in general or at least all mine do. Only if I feel two cycles are required do I do this though. Like when it's very cold out. Just to make sure you are not covering up for one bad plug though do the resistance check on each plug through the wiring harness. To be fair though I do not own a 603 engine type.