I would start with the glow plugs/module. I had a '92 that had a problem starting and it had 4 bad glow-plugs. Replaced them and no more problems.... If I remember correctly, they can be tested with an ohm-meter. Have you asked about this up in the 7.3 forum? Lots of experts there..... here's a link https://www.ford-trucks.com/forums/forum117/

Here's alink to the glow plug troubleshooting section. https://www.ford-trucks.com/forums/9...le-system.html