SUEMCVAY

When driving for over 1 hour on the motorway so averaging 70 mph the above notice comes up on my dash. I've had it in to Lexus for diagnostics 3 times but they are unsure what it could be. Even though it has the notice the car is running fine unless I go too far without stopping say overnight and then its ok again but still with the sign showing. Has anyone else had this problem and do you have any ideas?