Hyundai H-100 Starting problems

I have a 1998 H-100 with a diesel engine, and I am having starting problems.
1. When I turn the key, the dash lights go on.
2. When I turn the key, I can hear the starter
3. It does not help to spray starting fluid into the air intake
4. The battery has power

This is the strange part; sometimes I can start the car in the morning when it is cold, I will drive for awhile, and then park the car. When I come back, the engine is still warm but the car wont start. Then if I wait till the next morning, the car starts up again.

I am wondering how to tell if it is the fuel pump, or the fuel pump relay? Where is the fuel pump relay located on the Hyundai H-100 and how can I tell if it is bad? If it is the fuel pump, where is it located; in the fuel tank?

Also, are parts for the Hyundai H-1 interchangeable with parts for the H-100 (fuel pump in particular)?

Thanks