Diesel fuel has to have a minimum of 10% biofuel mixed in. Super market diesel can contain as much as 15%. As a result sludge/fungus can build up in the tank, lines and fuel filter which can cause misfires and hesitation. There are additives that dissolve this sludge/fungus. This happened to my mondeo ST tdci. Plus try adding mineral 2 stroke oil into the fuel. Mix ratio is about 300ml to 50litres of fuel.