Maybe the timing belt slipped a tooth? That would explain the loss of power and the huge plumes of smoke from the exhaust. Another possibility: an injector busted, and fuel started pouring into one of the cylinders. That happened to a friend's 240SX, with more/less similar symptoms. The cylinder will have zero compression, and the engine will chug as the other three are forced to compensate... and when the plug fires, the excess fuel will be ignited and dumped, en masse, out your exhaust. Just an idea.