My 1985 300D is becoming unreliable in starting. After warming the glow plugs turning the tumbler to "start" will not always engage the starter. Further, sometimes holding the tumbler in the "start" postion will have a noticeable delay in engaging the starter solenoid, if it happens at all. The battery is new and is fully charged. The starter was replaced just a few weeks ago with a new rebuilt Bosch. Is the problem in the tumbler, or is there some other relay that I don't know of?