my pegouet 406 stop's when it gets its speed down?

i've gotta a 406 pegouet model 2001 with a 2200 c.c motor,
the problem that the car stops when the speed goes down or as soon as i break with the breaks & the rpm sometimes get down & return back to its normal place sometimes the engine stop....also when i accelerate the car speed i found black Exhaust fumesز