Did some checking around the FAQ on the topic of sensors. I think we are on the right track. The engine coolant temperature sensor (ECT) looks like a likely suspect. The cost is not too $$$, about $42 Cdn. I will also clean out the air control valve (ACV) and throttle body while I'm at it. I'll start by checking the level of coolant first. I remember owning a Trooper that would surge continuosly at idle when warmed up. The coolant was down by less than a litre/quart, just enough to interfere with the proper operation of the sensor.