I have a 2002 Renault Cleo CDi which has developed an intermittent loss of power. I might be cruising along quite happily when suddenly the power drops. If I change down a gear the power will return followed by a white puff of smoke from the exhaust. There doesn't seem to be any particular pattern as to when the loss of power will occur. My mechanic has diagnosed an air leak in the system but has so far been unable to pinpoint it, even after rigging a temporary line from the fuel tank to the engine to see if the air leak is between the two. Does anyone have any ideas about where the problem might lie?