How can i fix intermittent over-revving?

My manual R reg (1997) Ford Mondeo 1.8 Turbo Diesel has started over revving to the extreme when moving slowly in heavy traffic. It does not stop when i take my foot of accelerator or put it in neutral- only switching off engine stops it. it does not happen every time its driven but once it happens once in the journey, it keeps happening if still trying to move in slow traffic. Lots of smoke comes out of exhaust but not engine, I thought it was white to start but the last time it happened 5 times in one journey and it was black smoke. i took it to a garage, they said its not the turbo and its not over-full of oil but the fault never occurred while it was there so they couldn't say for sure what it was! Any ideas?!!