My car stalls sporadically. When I rev the engine black smoke comes from the exhaust. I can also smell the exhaust in my car. I have had the whole exhaust system replaced including the muffler. Occasionally it will lose compression and will not accellerate, causing me to pull over to the side of the road and wait. I can usually turn it off and wait 5 minutes or so and it will then accelerate and I can drive away. What do you think the problem can be?