I own a 1998 Concorde with just under 136,000 miles. The car starts up and seconds after, the RPM drops and the car shuts off. It will do this for about 5 times then it will work. Sometimes when I'm at the stop sign or red light the car will just die, then start back up again. Also, while I'm driving on the highway the RPM drops as if it's going to die then if I give it gas the car will jerk then run again. I forgot to add that for about a week the bottom vents are always on high even if I have the AC/Heat off. I took it to my mechanic and he couldn't find anything wrong he said. Any help to what I can do to fix these problems? Thanks.