Might then be something temperature related as you noted. Could be the ECU sender is a bit off (reading too low a temp) and when you went wheeling, the temperature in the coolant went up a little and that was enough to kick the ECU out of fast idle mode. Or it may be the idle air valve is doing something similar. Or the thermostat may be too cold (or opening too soon) and that is making the engine run too cold.

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