Thanks for all the suggestions. I will check on the glow plugs this weekend. I do not think there is a load on the motor that is keeping it from turning over fast enough. I think the starter is original so it is 25 years old. But what I can"t figure out is how fast does the motor need to turn over to start?
Also I noticed that the ground wire off the battery gets warm when I am cranking it over for a time. All the connections are solid - what causes this and can the issues be related? Thanks again.