Ford Fiesta intermitant fault

Hi Im in need of some help. Im desperate. About 3 months ago I was driving on the motorway when I had to slow down, the engine started to lag, or chug, the engine management light came on and white smoke started to emit from the exhaust. THe next day I took it to a garage who couldnt find the fault. I then drove the car the next day and the problem had gone for a short while and then came back. A diagnostic shows misfire on one. I have had coils changed, leads changed, injectors checked, but no joy. I can drive the car all day and no problem or I can drive for a few hours and the judder starts, if I continue to drive the smoke emits from the exhaust and fuel consumption goes through the roof. The engine light can be either on, flashing, or off. The first sign of the problem is when I dip clutch and engage gear, the car will do a slight judder and then be ok after a second, but it will get worse and worse from then on. Then the next day no fault. PLease, please help I have had 3 mechanics on it all of which say different things.