This sound like an intermittent problem, and as you have started the elimination game, I know its not easy to find. Murphy strikes when least expected. It could be anything from a connector that has got moisture into it and makes and brakes intermittently. You need to check all your connectors even the least important ones. Unplug and replug computer box and inspect to see if there isnt any corrosion anywhere on any connectors. Sometimes due to vibration you might also have a component that has come lose on computer board in computer box (if this is not a sealed unit, I had this in remote fob) Also with high kilos on vehicle check to see if there is no spot where harness rubs on body or engine and possibly rubbed through and confused computer. Another place to focus on might be your immobilizer alarm unit, there might be a loose component. From your query you say it happens at around 3000rpm, seems this is a point where something resonates to bring up condition though not always. Hope you find the gremlin.
Anonymous