2002 Vauxhall Zafira engine light power loss

please help guys.. driving my diesel zaf all going great. all of a sudden the engine light comes on the dashboard then the power drops right down. to cure the problem i pull over , switch off, and restart. because the light is off the car starts ok but with huge cloud of smoke and we are away. it must be some sort of sensor or tube me thinks.. i know there is one clever person there who has seen this and knows the answer.. please help..many thanks for your time..js