Transit diesel, oil in air filter/ White smoke/misfiring?

Hi , i have a 99' Ford transit 2.5 (non turbo) diesel.

I have found oil in the air filter which is causing a little black smoke, but the real problem is when i try to drive over 50mph on the motorway i get huge plumes of white smoke and a loud rattling noise which both subside after a few seconds of reduced speed and revs. It has also has quite random patches of firing on three cylinders.

Any help would be greatly appreciated. Thanks