[retro]

The ignition switch check out ok too? Did you replace with OEM parts only? It could be something as simple as a dirty/corroded harness plug connection somewhere. Or an engine or frame ground corroded or loose...? Does the neutral indicator work?

 

[retro]

I don't have the service manual for the 98 450 ES, so I don't know if this is relevant or not... but does the oil temp control module shut down the ignition if it detects an overheated condition? If so, troubleshoot the oil temp sensor and module circuits.

Edit:
This is probably irrelevant, but does the display function like it should? Everything work on it? Doesn't reset itself or flicker the oil temp light?

 

[retro]

Do you have the service manual? If not, its a must-have for troubleshooting and you can get it HERE.

 

[retro]

I love to hear that! Thank you for letting us all know what you found wrong with it! Jolly good show...