Sounds like what happened to my engine. If it is in fact what happened to me, you have a leaking piston, meaning there's something leaking inside it; whether it be water or oil. If they can find the leak, they can try to patch it; BUT that rarely works. In other words, your engine is gone. That's right,i'm on my second engine. Anyways, if this symtom just started, you got like a good 5 miles of driving before it gets really bad. ALSO, check plugs on your distributor to see if any got loose.