Sounds like a vacuum leak or it could be plugged up exhaust. Did someone stick a potato in your tail pipe? If its a vacuum leak, you should be able to to find it. Do you have vacuum wipers? A stethoscope can be helpful, or just a piece of rubber tubing to narrow it down. If you absolutely can't find it, put something like Marvel Mystery oil in a pump oil can and squirt it around the manifold. A leak will suck it in and the exhaust will smoke.