2011 E550 goes into sport mode itself
 

Thank you for any advice, in advance.

The car has 40K miles and no warning indicators on. Periodically, I'll be driving in E mode an the car will switch to S mode by itself. The sport mode button is illuminated and I cannot switch back to E mode be pressing the button.

Unless I cycle the ignition, I cannot put back into E mode.

Any ideas?