False ESP warnings

On my ML-350 from oct 2005 I get false ESP warnings when I drive on the highway with a speed above 60 m/h. I feel that the ESP is working although there are no signs of a slippery road.  In the beginning it was only a few times during a drive. At this moment, a month later, it is more or less continuously. To drive normal you have to switch off the ESP. So,I checked the ESP with the Mercedes Star Diagnostic for fault codes but there where none. Also the event memory was empty. Do you have any idea what can causes the ESP warnings?

Best Regards,