Yes your ECU could be causing this.
Keep looking, you can get them cheaper than $500.

You never said what year your truck is, I know of a guy parting out a '96.

 

It's a 1998 discovery 4.0l.

 

I believe you would need a '97-'99 ecu

 

yup, one with AEL.

 

Now if I replace the ECM do I need to return to the dealer to have it reprogramed. The one Land Rover mechanic said it was a plug and play, the other thought it would have to be reprogramed. My son said he has had good luck with AutoECMS.com for a new one if I go that route.