Volkswagen Passat losing power traction control light on vibrating

A few months ago my Passat would intermitantly lose power and start vibrating when accelerating and the Traction Light would appear so after pulling over and turning off for a minute I could start again and drive normally only for it to happen again with no warning or reason. I was told many different reasons for this and have changed fuel filter, air filter and MAFM with no fixing of the problem. I then noticed black puffs of smoke from exhaust when is started juddering before totally losing power, I started putting Dipetene in tank when filling up and problem seemed to be getting better. Then as driving at 120 kph for an hour with no problems car just started violently juddering as before and lost power and light on but pulling over and turning off and on light still on and would shudder immediatley on starting and sounding really rough. Had to get recovered home and recovery driver told me sounded like only firing on 3 cylinders which sounds like to me too. Any Ideas as I am totally baffled as to why this is happening?