Brakeslast - Mercedes Benz GL500 5.7 Litre 2007 Model

Warning light (yellow triangle with !) flashes and power cuts out.
At first it was at highway speed only but now at low speed also.
It happens when I come off the throttle. The motor seems to die, about 65% loss of power but comes back if I stay off the throttle for a second or 2. The flashing light stops but returns in identical fashion as above.