OK well I have checked the reading against the dash hidden obd and another obd hand-held and the results are the same. Its as if the increased RPM forces the stat open and allows a large inrush of cold water. Running at 70 mph at light load, drop 2 gears and floor it, temp drops 15C in 5 to 8 seconds. Lift off and drop back to normal 'drive auto' and the lost temp rebuilds in about 20 seconds. All other temp readings during warm up etc look normal. No OBD fault codes are being logged.