I don't know from diesel but the symptoms sound similar to a defective MAF (Mass Air Flow) valve, a component of the air intake system. Very easily replaced since it is an external component, might cost a bit though. A common breakdown with 4.0 litre Ford motors (1.8 l motor in a van!--we don't see those in North America).    Similar to what David suggests, an air mixture problem.
However the fuel deposits (on your rear window??) don't fit with this guess; maybe your "mechanic" is on the right track (But remember, everything you observe is not necessarily related to the specific problem).

I Suggest you PM Catspaw49, he's the top-ranked gearhead on the Cat.

Good luck withthat.

W-O