Dribbling injectors? E34 525i-M20 cambelt engine.

[tetleysmooth]

Here I am again with another E34 525i question. I've been trying to suss out an enormous flatspot when cold.
I've replaced the afm, airfilter, plugs, leads and water temp sensor.
Now when I start the car it seems like it's overfuelling. It runs like a dog until I give it a good blip on the throttle and it blows a fair old cloud of black out of the exhaust, then it settles down.
The mixture is spot on now. The car passed the mot on Wednesday and the chaps set the mixture while it was hooked up to their gas analyzer.
I'm wondering if the injectors may be dribbling while the car's not in use.
Are these known for doing this?
It's an F reg car with the later Motronic injection system.
Thanks again people.