Q&A

Thank you Andrew. There were NO fault codes evident from the On-Board Diagnostic (OBD-II) system. (Assuming that's what "DTCs" means ?) The 'flame trap' has been cleaned, and any vacuum leaks searched-for, and the Coolant Temperature Sensor has been replaced. There must be a faulty electronic feed-back loop when the Throttle starts Loping up and down while we are cruising along in Drive. Weather mild, and even in rain about 14 degrees C. Initially we thought it was related to engine getting hot climbing long steep grades, but since then the fault's happened at modest workloads/speeds intermittently. We apparently have a Mechanic's nightmare ?!? as "good luck" has meant running on a Dynamometer to get the Loping in a workshop. Any other suggestions very welcome - we'll get them to look at the Air Mass Meter, etc.
Specifically, does an ECU need to 'follow' its OME-engine - could there possibly be an incompatibility between ECU and Engine ?? The vehicle may have been re-engined (still Non-Turbo before and after). Thanks again.

Well, thanks for those suggestions, but it turned out that the IDLE AIR CONTROL VALVE was the intermittently faulty link in the [broken] chain. Took a couple of returns to our Mechanics to pinpoint the source of this Loping While Driving problem, but fortunately they are very persistent and thorough.